boot: Extend `etc/show-boot-menu` to configure skipping boot menu with only one device

boot: Extend `etc/show-boot-menu` to configure skipping boot menu with only one device

Concerns were raised, that skipping the boot menu, if only one device is
present, might make debugging issues more difficult. So, extend the
current runtime configuration option `etc/show-boot-menu` to enable this
feature by setting it to 2.

Fixes: 29ee1fb8 ("Skip boot menu and timeout with only one boot device")
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>