test/py: android: extend abootimg test

test_abootimg is extended to include the testing of boot images
version 4. For this, boot.img and vendor_boot.img have been
generated using mkbootimg tool with setting the header
version to 4.

This tests:
- Getting the header version using abootimg
- Extracting the load address of the dtb
- Extracting the dtb start address in RAM

Running test:
$ ./test/py/test.py --bd sandbox --build -k test_abootimg

Signed-off-by: Safae Ouajih <souajih@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>