admin: Add API to OtBootloader to get name

https://bugzilla.gnome.org/show_bug.cgi?id=706548
admin: notify detected bootloader configuration

OSTree now supports multiple bootloader backends so
notify which bootloader configuration was detected.

https://bugzilla.gnome.org/show_bug.cgi?id=706548

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>