Define, update and move get_secureboot_mode() to a common file for use
by other tests.

Updated to check both the efivar SecureBoot-$(UUID) and
SetupMode-$(UUID), based on Dave Young's review.
Signed-off-by: NMimi Zohar <zohar@linux.ibm.com>
Reviewed-by: NPetr Vorel <pvorel@suse.cz>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>