main/lk2nd: new package (MR 2570)

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 package is split
into 2 subpackages, each of them installs `/boot/lk2nd.img`.
So the end result is - lk2nd-msm8916 and lk2nd-msm8974 can't
be installed at the same time.

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 devices using only pmbootstrap tool.
At the very least, pmbootstrap can suggest user to flash lk2nd
immediately after `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 ever want this.