classes/kernel-fitimage: add ability to add additional signing options

classes/kernel-fitimage: add ability to add additional signing options

Add a UBOOT_MKIMAGE_SIGN_ARGS variable to enable passing additional
options to uboot-mkimage when it is run the second time to perform
signing.

Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 8fd7ee7414b45a1feeef7982af3583475902a677)
Signed-off-by: Massimiliano Minella <massimiliano.minella@se.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>