oe-selftest: fitimage: Add test for signing U-Boot FIT image without SPL

oe-selftest: fitimage: Add test for signing U-Boot FIT image without SPL

This adds a new selftest case `test_sign_uboot_fit_image_without_spl` to verify
that the build can correctly generate and sign a U-Boot FIT image in a scenario
where no SPL is used.

Background:
- Some boards build only the U-Boot proper FIT image and do not require an SPL.
- The signing flow must handle this case gracefully: generate the ITS, sign
  the FIT image, and skip signing/injecting a key into the SPL DTB.

What this test does:
1) Enables `UBOOT_FITIMAGE_ENABLE` and `SPL_SIGN_ENABLE` but explicitly sets
   `SPL_DTB_BINARY` to an empty string to indicate that no SPL is present.
2) Verifies that the U-Boot ITS and FIT image are built successfully.
3) Confirms that the generated ITS file includes signature metadata as requested.
4) Dumps the FIT image to ensure that the signature nodes exist.
5) Confirms that the log for `do_uboot_assemble_fitimage` shows the expected
   mkimage/mkimage_sign invocation.

This ensures that signing works correctly even when only the U-Boot proper is built,
which matches real-world configurations that do not require an SPL.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>