test/py: android: Add test for abootimg

Unit test for 'abootimg' command. Right now it covers dtb/dtbo
functionality in Android Boot Image v2, which was added recently.

Running test:

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

shows that 1/1 tests passes successfully.

Signed-off-by: Sam Protsenko <joe.skb7@gmail.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>