oeqa/selftest/fitimage: update tests to use two keys

Other keys (UBOOT_SIGN_IMG_KEYNAME) are required to sign individually the images
nodes, and these keys must be different from the ones used to sign the
configurations nodes (UBOOT_SIGN_KEYNAME), then fitimage tests need to be
updated.

Signed-off-by: Thomas Perrot <thomas.perrot@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>