crosshatch: package current mainline kernel attempt (#153)

This packages my current kernel from #153 (closed). Also obsoletes !103 (closed).

This is tested to boot on google-crosshatch, following these instructions:

• pmbootstrap init, choose the mainline kernel
• pmbootstrap install --no-fde
• pmbootstrap initfs hook_add debug-shell
• enter fastboot
• fastboot set_active a
• fastboot flash dtbo_a ~/.local/var/pmbootstrap/chroot_rootfs_google-crosshatch/boot/dtbo-google-crosshatch-mainline.img
• pmbootstrap flasher boot

and you should get telnet after a while.

This kernel takes a long time to build, so I cancelled the CI job. I can confirm it builds locally.