postmarketos-mkinitfs: Move Android kernel update post processing to separate repo

postmarketOS have put a lot of time into building scripts that are capable of post-processing the Linux kernel to create the necessary boot images, ramdisk etc, as other distros (ALARM, void) are starting to see more support for Android mainline devices, I think it would greatly increase the value of these scripts if we split them into a separate git repo and made them accessible to other distros.

It seems like other distributions are also adopting the deviceinfo format, making this even more feasible / sensible.