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>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index fc018bd..790dcb0 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -25,6 +25,7 @@
 CONFIG_CMD_CPU=y
 CONFIG_CMD_LICENSE=y
 CONFIG_CMD_BOOTZ=y
+CONFIG_CMD_ABOOTIMG=y
 # CONFIG_CMD_ELF is not set
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_GREPENV=y