osbuild: rename qemu_stage_buildroot to host_as_buildroot

It's now used for more than just the qemu stages so let's
rename it to be slightly more appropriate.

src/osbuild-manifests/coreos.osbuild.aarch64.mpp.yaml

@@ -34,12 +34,15 @@ mpp-vars:
     mpp-if: osname in ['rhcos', 'scos']
     then: "name:deployed-tree"
     else: ""
-  # For the qemu stages we'll use the host buildroot (COSA)
-  # because we definitely don't include qemu-img in CoreOS. ""
+  # For some stages (i.e. the qemu stages) we'll use the host as
+  # the buildroot (i.e. COSA in most cases but sometimes just
+  # the actual HOST filesystem like in coreos/custom-coreos-disk-images).
+  # This is useful/necessary because we definitely don't include
+  # qemu-img or tools like zip in the actual CoreOS OS. "" here
   # means to use the host as buildroot. It is worth noting that
   # the host buildroot is the default if nothing is specified.
   # We're still defining it here in an attempt to be explicit.
-  qemu_stage_buildroot: ""
+  host_as_buildroot: ""
 mpp-define-images:
   - id: image
     sector_size:

src/osbuild-manifests/coreos.osbuild.ppc64le.mpp.yaml

@@ -34,12 +34,15 @@ mpp-vars:
     mpp-if: osname in ['rhcos', 'scos']
     then: "name:deployed-tree"
     else: ""
-  # For the qemu stages we'll use the host buildroot (COSA)
-  # because we definitely don't include qemu-img in CoreOS. ""
+  # For some stages (i.e. the qemu stages) we'll use the host as
+  # the buildroot (i.e. COSA in most cases but sometimes just
+  # the actual HOST filesystem like in coreos/custom-coreos-disk-images).
+  # This is useful/necessary because we definitely don't include
+  # qemu-img or tools like zip in the actual CoreOS OS. "" here
   # means to use the host as buildroot. It is worth noting that
   # the host buildroot is the default if nothing is specified.
   # We're still defining it here in an attempt to be explicit.
-  qemu_stage_buildroot: ""
+  host_as_buildroot: ""
 mpp-define-images:
   - id: image
     sector_size:

src/osbuild-manifests/coreos.osbuild.s390x.mpp.yaml

@@ -38,12 +38,15 @@ mpp-vars:
     mpp-if: osname in ['rhcos', 'scos']
     then: "name:deployed-tree"
     else: ""
-  # For the qemu stages we'll use the host buildroot (COSA)
-  # because we definitely don't include qemu-img in CoreOS. ""
+  # For some stages (i.e. the qemu stages) we'll use the host as
+  # the buildroot (i.e. COSA in most cases but sometimes just
+  # the actual HOST filesystem like in coreos/custom-coreos-disk-images).
+  # This is useful/necessary because we definitely don't include
+  # qemu-img or tools like zip in the actual CoreOS OS. "" here
   # means to use the host as buildroot. It is worth noting that
   # the host buildroot is the default if nothing is specified.
   # We're still defining it here in an attempt to be explicit.
-  qemu_stage_buildroot: ""
+  host_as_buildroot: ""
 mpp-define-images:
   - id: image
     sector_size:

src/osbuild-manifests/coreos.osbuild.x86_64.mpp.yaml

@@ -34,12 +34,15 @@ mpp-vars:
     mpp-if: osname in ['rhcos', 'scos']
     then: "name:deployed-tree"
     else: ""
-  # For the qemu stages we'll use the host buildroot (COSA)
-  # because we definitely don't include qemu-img in CoreOS. ""
+  # For some stages (i.e. the qemu stages) we'll use the host as
+  # the buildroot (i.e. COSA in most cases but sometimes just
+  # the actual HOST filesystem like in coreos/custom-coreos-disk-images).
+  # This is useful/necessary because we definitely don't include
+  # qemu-img or tools like zip in the actual CoreOS OS. "" here
   # means to use the host as buildroot. It is worth noting that
   # the host buildroot is the default if nothing is specified.
   # We're still defining it here in an attempt to be explicit.
-  qemu_stage_buildroot: ""
+  host_as_buildroot: ""
 mpp-define-images:
   - id: image
     sector_size:

src/osbuild-manifests/platform.hyperv.ipp.yaml

@@ -53,7 +53,7 @@ pipelines:
              target: /boot
   - name: raw-hyperv-image-vhdx
     build:
-      mpp-format-string: '{qemu_stage_buildroot}'
+      mpp-format-string: '{host_as_buildroot}'
     stages:
       - type: org.osbuild.qemu
         inputs:
@@ -70,7 +70,7 @@ pipelines:
             type: vhdx
   - name: hyperv
     build:
-      mpp-format-string: '{qemu_stage_buildroot}'
+      mpp-format-string: '{host_as_buildroot}'
     stages:
       - type: org.osbuild.zip
         inputs:

src/osbuild-manifests/platform.qemu-secex.ipp.yaml

@@ -353,7 +353,7 @@ pipelines:
                 mpp-format-int: '{image_secex.layout[''roothash''].size}'
   - name: qemu-secex
     build:
-      mpp-format-string: '{qemu_stage_buildroot}'
+      mpp-format-string: '{host_as_buildroot}'
     stages:
       - type: org.osbuild.qemu
         inputs:

src/osbuild-manifests/platform.qemu.ipp.yaml

@@ -82,7 +82,7 @@ pipelines:
               target: /boot
   - name: qemu
     build:
-      mpp-format-string: '{qemu_stage_buildroot}'
+      mpp-format-string: '{host_as_buildroot}'
     stages:
       - type: org.osbuild.qemu
         inputs: