image: add do_image_qa task to run QA checks on the constructed image

image: add do_image_qa task to run QA checks on the constructed image

This task runs all functions in IMAGE_QA_COMMANDS after the image
construction has completed in order to validate the resulting image.

Image sanity checks should either be Python functions which raise
bb.build.FuncFailed on failure or shell functions with return a
non-zero exit code.

Python functions may instead raise an oe.utils.ImageQAFailed
Exception which takes an extra argument, a description of the
failure.

   python image_check_python_ok () {
       if True:
           raise bb.build.FuncFailed('This check always fails')
       else:
           bb.note("Nothing to see here")
   }

   image_check_shell_ok () {
       if true
           exit 1
       else
           exit 0
       fi
   }

[YOCTO #9448]

Signed-off-by: Joshua Lock <joshua.g.lock@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>