test/py: Add base test case for A/B updates

Add sandbox test for 'ab_select' command.

Test: ./test/py/test.py --bd sandbox --build -k test_ab

Signed-off-by: Ruslan Trofymenko <ruslan.trofymenko@linaro.org>
Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Alistair Strachan <astrachan@google.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 61391a7..11cc097 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -20,6 +20,7 @@
 CONFIG_LOG_MAX_LEVEL=6
 CONFIG_LOG_ERROR_RETURN=y
 CONFIG_DISPLAY_BOARDINFO_LATE=y
+CONFIG_ANDROID_AB=y
 CONFIG_CMD_CPU=y
 CONFIG_CMD_LICENSE=y
 CONFIG_CMD_BOOTZ=y
@@ -47,6 +48,7 @@
 CONFIG_CMD_SPI=y
 CONFIG_CMD_USB=y
 CONFIG_CMD_AXI=y
+CONFIG_CMD_AB_SELECT=y
 CONFIG_CMD_TFTPPUT=y
 CONFIG_CMD_TFTPSRV=y
 CONFIG_CMD_RARP=y