Draft: purism-librem5: use extlinux instead of u-boot script

This will simplify boot process and allow to boot with UUIDs

Draft because it depends on next boot-deploy release

CC @craftyguy @Newbyte @QC8086 @panzersajt @antonok (maintainers and testing team)

For testers: Using mrtest should be good enough, but if it doesn't work as intented it may break booting, so use clean installation or do a backup