svn no longer works with github.com for raspberrypi/firmware

[vanillaSprinkles]

first discovered while working with netboot.xyz (issue posted at netbootxyz/netboot.xyz#1421)

the makefile needs a little rework around https://github.com/ipxe/pipxe/blob/master/Makefile#L29
related github blog post on svn deprecation: https://github.blog/2023-01-20-sunsetting-subversion-support/

currently diving down the rabbit hole of some fixes; but i believe the sparse-checkout is the way to go
Edit: i ended up updating the sub-modules to reflect what the netboot.xyz has done; i've a WIP pull request on my fork; need to have it output the sdcard images so it can be tested by more than me: vanillaSprinkles#1
edit2: used that branch to output a release of img and zip's; extracted both zip files and did a simple diff on the 2x folders; below are results (i do not yet have a means of testing them booting in an rpi):

find sdcard_rpi{3,4} -type f | sed -r 's/^sdcard_rpi[34]//g' | sort -u  | xargs -I{} diff sdcard_rpi3/{} sdcard_rpi4/{}
Binary files sdcard_rpi3//RPI_EFI.fd and sdcard_rpi4//RPI_EFI.fd differ

[ below can be ignored since pull request has been made in my own branch and am testing it out ]
my current fix includes (will make a pull request once I get this ironed out):

(top of file)

FW_REPO_URL     := https://github.com/raspberrypi/firmware
FW_BRANCH       := stable
FW_SUBDIR       := boot

(submodules to handle later gcc versions aborting build - this is not related to the svn fix, but is a hack to get the current state building)

submodules :
        git submodule update --init --recursive
        sed -i -r 's@(-nostdlib)( -g)@\1 -Wno-vla-parameter -Wno-stringop-overflow -Wno-use-after-free -Wno-dangling-pointer\2@g'   edk2/BaseTools/Source/C/Makefiles/header.makefile

(firmware block)

firmware :
        if [ ! -e firmware ] ; then \
                $(RM) -rf rpi_firmware ; \
                git clone --depth 1 --no-checkout --branch $(FW_BRANCH)  '$(FW_REPO_URL)'   rpi_firmware ; \
                cd rpi_firmware ; \
                git config core.sparseCheckout true ; \
                git sparse-checkout set $(FW_SUBDIR) ; \
                git checkout ; \
                cd - ; \
                mv rpi_firmware/$(FW_SUBDIR) firmware ; \
        fi

(clean block)

clean :
        $(RM) -rf firmware rpi_firmware Build sdcard sdcard.img sdcard.zip
        if [ -d $(IPXE_SRC) ] ; then $(MAKE) -C $(IPXE_SRC) clean ; fi