PureBoot doesn't have any other three-valued settings and this doesn't
present very well in the config UI.

Instead make this a two-valued setting; drop the mode that forces the
EC setting to "stay off" at every boot because this is the default.

When disabling automatic power-on, disable the EC BRAM setting too.

Signed-off-by: Jonathon Hall <jonathon.hall@puri.sm>