image: simplify Linksys footer creation and fix alignment

Current factory image sizes for Linksys devices are 256-byte aligned, while
OEM FW images appear aligned with the 128K erase blocksize. This is not an
issue writing factory images from the OpenWrt or Linksys GUIs, but can
lead to failures when using a TFTP client from the Linksys bootloader.

Simplify Linksys footer creation by migrating to a makefile build recipe,
and add footer pre-padding to ensure the proper final image size. Finally,
remove the linksys-image.sh script no longer needed.

Link: openwrt#11405 (comment)
Link: openwrt#11405 (comment)

Signed-off-by: Tony Ambardar <[email protected]>

include/image-commands.mk

@@ -383,11 +383,30 @@ define Build/kernel-bin
 	cp $< $@
 endef
 
+# Linksys image footer (256 bytes) is appended to the factory image and tested
+# by both the Linksys Upgrader (as observed in civic) and OpenWrt sysupgrade.
+# Final file is $BLOCKSIZE aligned to satisfy some devices' TFTP clients, by
+# preceding the footer with padding (0xFF).
+# Footer format:
+#  .LINKSYS.        Checked by Linksys upgrader before continuing.  (9 bytes)
+#  <VERSION>        Upgrade version number, unchecked so arbitrary. (8 bytes)
+#  <TYPE>           Model of device, space padded (0x20).          (15 bytes)
+#  <CRC>            CRC checksum of factory image to flash.         (8 bytes)
+#  <padding>        Padding ('0' + 0x20 * 7)                        (8 bytes)
+#  <signature>      Signature of signer, unchecked so arbitrary.   (16 bytes)
+#  <padding>        Padding with nulls (0x00)                     (192 bytes)
 define Build/linksys-image
-	$(TOPDIR)/scripts/linksys-image.sh \
+	let \
+		size="$$(stat -c%s $@)" \
+		pad="$(subst k,* 1024,$(BLOCKSIZE))" \
+		offset="256" \
+		pad="(pad - ((size + offset) % pad)) % pad"; \
+		dd if=/dev/zero bs=$$pad count=1 | tr '\000' '\377' >> $@
+	printf ".LINKSYS.01000409%-15s%08X%-8s%-16s" \
 		"$(call param_get_default,type,$(1),$(DEVICE_NAME))" \
-		$@ $@.new
-		mv $@.new $@
+		"$$(cksum $@ | cut -d ' ' -f1)" \
+		"0" "K0000000F0246434" >> $@
+	dd if=/dev/zero bs=192 count=1 >> $@
 endef
 
 define Build/lzma