add control for generic grub pc via serial line
Implementation [YOCTO #5615].
(From OE-Core rev: 01968e9244d0cf3deb1ec5cfb8e562d3b364add6)
Signed-off-by: Cristian Iorga <cristian.iorga@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>