selftests/kexec: update get_secureboot_mode

The get_secureboot_mode() function unnecessarily requires both
CONFIG_EFIVAR_FS and CONFIG_EFI_VARS to be enabled to determine if the
system is booted in secure boot mode.  On some systems the old EFI
variable support is not enabled or, possibly, even implemented.

This patch first checks the efivars filesystem for the SecureBoot and
SetupMode flags, but falls back to using the old EFI variable support.

The "secure_boot_file" and "setup_mode_file" couldn't be quoted due to
globbing.  This patch also removes the globbing.

Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>