systemd: support systemd-boot as a stand-alone EFI bootloader

systemd: support systemd-boot as a stand-alone EFI bootloader

The "systemd-boot" is gummiboot now included into systemd project.
The old gummiboot project supported in OE is dead.

Our intention is to get a gummiboot-like EFI bootloader without
much dependency on systemd and its features.

This work is largely derived from the existing bbclass and recipes
of gummiboot and systemd.
(commit tip: ee25d0e3987d7732a2e46e1640693b4cf419a9fc)

Please refer to the history up to the tip for authorship and
credit information for the original works.

To enable the systemd-boot in build, add this line
EFI_PROVIDER = "systemd-boot" in your machine conf file.

Signed-off-by: Jianxun Zhang <jianxun.zhang@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit e9add1cd01e498d2aa52528ec52342cae48a387a)
Signed-off-by: Andreas Oberritter <obi@opendreambox.org>