droid4-kexecboot buildroot source
It works by using utagboot generated mmcblk1p8 utags partition to boot the stock kernel to a 4MB buildroot image that can be flashed to the unused mmcblk1p13 bpsw partition. Then it starts kexecboot to use kexec for booting to the selected distro.
Currently supported are recent mainline kernels with the pending LCD patches added. SafeStrap is not necessary but can be booted to. And v3.0.8 legacy kernels installed into SafeStrap rom slots are supported (but need to be manually updated if slot kernels are updated) .
UPDATED: Booting to the stock Android and SafeStrap is now supported out of the box with for pivot_root.
Changes are needed to v3.0.8 kernel config and initramfs to not use CONFIG_CMDLINE_FORCE=y and safestrap/active_slot file and instead just use kernel cmdline to mount the desired rootfs. Distros using the mainline kernel do not have these limitations, and can use the 608MB mmcblk1p22 preinstall, 1.3GB mmcblk1p23 webtop partitions. And the 8GB mmcblk1p25 can also be reformatted to ext4 and used as rootfs if you don't need it for Android. I have not dared to try to reformat mmcblk1p20 system partition, but that too might be usable unless the Motorola bootloader does some checks on it on boot.
There are some experimental patches for initramfs modifications for postmarketos:
But we need also similar Android changes if somebody has time to work on patching and rebuilding LineageOS