package/systemd: enable building of systemd-boot

package/systemd: enable building of systemd-boot

systemd-boot is the integration of gummiboot into systemd, when
gummiboot is no longer maintained [0].

Add an option to build systemd-boot as part of the systemd build.

Install the boot files, that can serve as a template for the user
to tweak for their system.

[0] https://cgit.freedesktop.org/gummiboot/commit/?id=55df1539c9d330732e88bd196afee386db6e4a1d

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
[yann.morin.1998@free.fr:
  - add missing depends on i386 || x86_64
  - add missing dependency to gnu-efi
  - add missing boot files
]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>