main/lk2nd: new package

Package secondary LK bootloader for some Qualcomm devices (for now: msm8916, msm8974, msm8226).

Bootloader binary is built from source in x86_64 chroot. Two targets are built in a sequence. Then the package is split into 2 subpackages, each of them installs /boot/lk2nd.img, so as result lk2nd-msm8916 and lk2nd-msm8974 can't be installed together in one chroot.

device-* packages then can depend on specific subpackage of this package and have proxy-bootloader image ready to use in /boot.

This can improve user experience with installing postmarketOS on several msm devices using only pmbootstrap tool. At the very least, pmbootstrap can suggest user to flash lk2nd immediately after doing pmbootstrap install or even offer some kind of pmbootstrap flasher integration.

This will also open possibilities to automatically update and flash lk2nd using package manager, it we will ever want this.