oe-selftest: fitimage: add test for signing FIT images

Add a new test to verify signing FIT images. Also includes testing for
the newly introduced FIT_SIGN_INDIVIDUAL, UBOOT_MKIMAGE,
UBOOT_MKIMAGE_SIGN, and UBOOT_MKIMAGE_SIGN_ARGS variables.

Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>