samsung-klte: switch to fastboot and require lk2nd

To make panel driver upstreamable to mainline linux, we need to use proper panel selection in secondary bootloader (still TODO), and therefore lk2nd will be strictly required to boot mainline port.

Switch flashing method to fastboot, enable sparse rootfs image generation, and depend on lk2nd package.

Depends on: postmarketOS/pmbootstrap!2174, to provide full integration of lk2nd in pmbootstrap.

So, full flashing procedure for samsung-klte will look like:

pmbootstrap init
pmbootstrap install
# connect phone in stock Samsung download mode
pmbootstrap flasher --method=heimdall-bootimg flash_lk2nd
# reboot holding volume_down to fastboot/lk2nd mode
pmbootstrap flasher flash_kernel --no-install
pmbootstrap flasher flash_rootfs
fastboot reboot

(first commit will go away when !3043 (merged) is merged)