[hammerhead] Make `pmbootstrap flasher flash_kernel` work

Created by: ollieparanoid

Hi @dzervas and @MartijnBraam,

amazing progress with hammerhead! Wifi is working? Wow! :D

So there is pmboostrap flasher boot, which should do the same as the command specified in the wiki: fastboot boot /mnt/rootfs_lg-hammerhead/boot/vmlinuz-lg-hammerhead /mnt/rootfs_lg-hammerhead/boot/initramfs-lg-hammerhead

Does that work?

(Also, can you upload photos and maybe share them on /r/postmarketOS?)