Disallow unsigned kernels if UEFI Secure Boot is enabled

If UEFI Secure Boot is enabled and kernel signature verification fails, do not
boot the kernel. Before this change, if kernel signature verification failed
then GRUB would fall back to calling ExitBootServices() and continuing the
boot.

Patch-Name: linuxefi_disable_sb_fallback.patch

Signed-off-by: Linn Crosetto <linn@hpe.com>