selftest/imagefeatures: Enable sanity test for IMAGE_GEN_DEBUGFS

Add new testcase to check IMAGE_GEN_DEBUGFS. Test makes
sure that debug filesystem is created accordingly. Test also check
for debug symbols for some packages as suggested by Ross Burton.

[YOCTO #10906]

Signed-off-by: Humberto Ibarra <humberto.ibarra.lopez@intel.com>
Signed-off-by: Yeoh Ee Peng <ee.peng.yeoh@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>