kernel-fit-image: Split signing variables

kernel-fit-image: Split signing variables

Right now all signing is done with a single variable: UBOOT_SIGN_ENABLE.
This has the side effect of not allowing for signing the fitImage while
not signing the uboot files.

This patch creates three new variables specific to FIT_KERNEL and
defaults them to the corresponding UBOOT variables.  That way all
existing code will remain the same, but we can selectively control just
signing the fitImage without also signing the uboot files.

Signed-off-by: Ryan Eatmon <reatmon@ti.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>