wic: add systemd-boot option in EFI boot image plugin
The new systemd-boot enabled in OE is the old gummiboot
merged into systemd project. Our intention is to replace
gummiboot with systemd-boot in OE once every feature based
on gummiboot is supported with systemd-boot.
Before we can purge gummiboot, we temporarily keep both of
the two bootloaders supported.
Patch doesn't do replacement for every "gummi" occurrence.
We think cleaning can be done in background after we disable
people to use gummiboot, so we just keep change small and
safe this time.
Signed-off-by: Jianxun Zhang <jianxun.zhang@linux.intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
