oe-selftest: fitimage: add testcases to test ATF and TEE

oe-selftest: fitimage: add testcases to test ATF and TEE

Add "test_uboot_atf_tee_fit_image" test caste to check u-boot FIT image and
Image Tree Source(ITS) are built and the ITS has the correct fields.

Add "test_sign_standalone_uboot_atf_tee_fit_image" test case to check if u-boot
FIT image and Image Tree Source (ITS) are created and signed correctly for the
scenario where only the u-boot proper fitImage is being created and signed.

Currently, ATF and TEE(optee-os) recipes are placed in meta-arm layer.
OpenEmbedded-Core is a basic and core meta layer. To avoid OpenEmbedded-core
depends meta-arm, both test cases are used dummy images for testing.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>