sd-boot: add auto-reboot and auto-poweroff entries

sd-boot: add auto-reboot and auto-poweroff entries

Currently only an auto-reboot-to-firmware entry is available. For other
features - like reboot and power off - one needs to press the uppercase
B and O respectively.

Embedded devices may be missing a full fledged keyboard, so allow for
sd-boot to generate those entries.

v2:
 - add to the config parser/man/bootctl/sd-boot info screen
 - keep them off by default
 - add the (O)ff and re(B)oot help text if boot entries are not shown
 - drop irrelevant get_os_indications_supported() comment
 - s/ShutDown/Shutdown/

v3:
 - cast shutdown_system() reboot_system() to void

v4:
 - shutdown -> poweroff
 - add trailing ",ignoring" in parser message
 - drop explicit default state assignment to "false"

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>