arm-bsp-extras/linux-yocto: Introduce support for UEFI Secure Boot

Create linux-yocto-uefi-secure-boot.inc file to sign
the kernel image.

Modify linux-yocto%.bbappend file to include
linux-yocto-uefi-secure-boot.inc file.

Signed-off-by: Amr Mohamed <amr.mohamed@arm.com>