@hexdump
Thank you for your work on these chain loaders. It has enabled me to do something I have wanted for a long time.
I have your g12a-u-boot.bin (renamed to u-boot.ext) running on my Ugoos AM6 plus, in a multi-os boot setup, with all OSes installed on the SD (utilizing the vendor boot for the all boot stages except the last, which is in u-boot.ext). Currently, I can boot into Armbian Meson64 (Hirsute flavor, built in QEMU based on the rootfs file tree), Manjaro ARM (for VIM3/Ugoos, copied from distro image), and a Armbian Buster mini as rescue (also from QEMU/rootfs).
By writing the entries in extlinux.conf according to the correct menu syntax (inferred from e.g. pxe_utils.c in u-boot source, see below) one can select which kernel+initrd+dtb to load, and which root file system to mount. (One has to be quick at the second prompt to make the OS selection. This prompt appears at the sysboot stage, after u-boot autoload has timed out. Perhaps the sysboot timeout can be extended a bit? Edit: The TIMEOUT keyword does not seem to take effect.)
On all the OSes I have kept their original /boot dir and then added a new mount point /boot_FAT32 where the first FAT32 partition is located (with extlinux.conf, kernels+initrds+dtbs, and u-boot.ext), and updated the local /etc/fstab accordingly. Then it is simple to propagate the changes made to /boot under a running OS also to the boot partition. The SD layout/geometry is essentially: From the first available sector (2048, no gap needed) and 300M onwards; the kernel+initrd+dtbs and extlinux.conf stuff on a FAT32 partition. The follows, after perhaps a common swap partition, a number of ext4 partitions with the different OSes.
Here is what the extlinux.conf can look like
P.S. I know it is hard to compile these loaders correctly: From u-boot-amlogic (custodian) have have managed to compile, starting with different defconfigs, chain loaders that boot but with; no (HDMI) screen, green screen (several flavors), and indeed, purple-pinkish screen, but not one single working properly. Doh.