Draft: Create a generic (universal) image that works on all platforms with u-boot on eMMC

01:07 < josch> minute: another idea for wasting less space on many platform-specific disk images is to 
               ship a universal image which makes use of the same mechanism that the debian-installer 
               sd-card makes use of, namely u-boot setting the ${fdtfile} variable:
01:07 < josch> https://source.mnt.re/reform/reform-system-image/-/merge_requests/104
01:07 < josch> the image created by this MR will boot on all platforms that have u-boot on eMMC
01:07 < josch> it does not come with its own u-boot as that would make the image device-specific
01:08 < josch> so if you flash the image generated by the CI pipeline for this MR to an sd-card, it 
               should boot fine on the pocket-reform
01:08 < josch> it will likely not boot on a311d as that one usually does not have u-boot on emmc
01:09 < josch> if you want to make that image boot on a311d you can write the image to an sd-card and 
               then write u-boot to the beginning (for example by using "reform-flash-uboot sd")

This change requires the ${fdtfile} variable set in u-boot so on the one platform for which these images are useful (because u-boot is always read from eMMC) this cannot work because the u-boot version that the imx8mplus in the pocket reform ships with does not set that variable... This is fixed in reform-imx8mp-uboot!1 (merged)