Automation test is disabled by default. User need set TESTCLASS
to qemu in conf/local.conf and run bitbake command "bitbake
poky-image-xxx" or "bitbake poky-image-xxx -c qemuimagetest" to
trigger it. Currently only the sanity test with two testcases are
added.
To run the test, user need prepare a testing environment:
1) "expect" should be installed on system
2) NOPASSWD should be set for user to run bitbake
