efi_loader: variable: add VendorKeys variable

The following variable is exported as UEFI specification defines:
VendorKeys: whether the system is configured to use only vendor-provided
	    keys or not
The value will have to be modified if a platform has its own way of
initializing signature database, in particular, PK.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>