oe-selftest: Add U-Boot fitImage signing testcases

oe-selftest: Add U-Boot fitImage signing testcases

Derived from the similar kernel fitImage sign testcase, the U-Boot
fitImage testcases exercises the following fitimage.FitImageTest
scenarios:

 * test_uboot_fit_image - create unsigned U-Boot fitImage
 * test_uboot_sign_fit_image - create unsigned U-Boot fitImage in
   addition to signed Kernel fitImage
 * test_sign_standalone_uboot_fit_image - Create signed U-Boot fitImage
   without a Kernel fitImage
 * test_sign_cascaded_uboot_fit_image - Create and sign U-Boot and
   Kernel fitImages

Signed-off-by: Klaus Heinrich Kiwi <klaus@linux.vnet.ibm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>