oeqa/selftest/imagefeatures: Add testcase for fitImage

oeqa/selftest/imagefeatures: Add testcase for fitImage

This testcase generates the Image Tree Source and
the corresponding fitImage containing a kernel and
a ramdisk. It then checks if the these files exist
and if the right fields are present in the right
order in the Image Tree Source.

Tested with: oe-selftest -r  imagefeatures.ImageFeatures.test_fit_image

Signed-off-by: Usama Arif <usama.arif@arm.com>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>