boot: remove unused boot_devices_bitmap variable

In addition to removing the variable, this also renames the parse_bootdevices()
function to validate_bootdevices(), as we don't need its return value anymore.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Acked-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
