device-google-{asurada,cherry,corsola,kukui,trogdor}: add systemd-boot

Ingo Reitzrequested to merge
9lore/pmaports:cros-bls into master

This adds support to boot on custom firmware using UEFI or LinuxBoot (kexec via boot loader spec detection)

Please note that all bootloaders that are anticipated to run support ext4 ESP partitions

Edited by Ingo Reitz

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