This patch enables usage of key id for private keys
in slimboot repo. Key ids are configured in
BuildLoader and platform BoardConfig files.
SLIMBOOT_KEY_DIR is set to default folder outside
sblopen.
Generation of extrenal Keyhash OS key hash to be configured
for QEMU/CGL/APL with appropriate keys.
Signed-off-by: Subash Lakkimsetti <subash.lakkimsetti@intel.com>