oe-selftest: fitimage drop test-mkimage-wrapper

oe-selftest: fitimage drop test-mkimage-wrapper

Rather than writing hints into log files and verify the hints can be
found, the tests should verify that the artifacts in the deploy folder
are correctly signed. This is a much better test.
u-boot-tools provide a utility fit_check_sign which can verify the
signatures in fit images. Lets use it.

grepping in temp/run. or temp/log. files also does not work if the tasks
runs from sstate and the corresponding run file is not even generated.

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>