oe-selftest: fitimage add more kernel tests

oe-selftest: fitimage add more kernel tests

* Test with only one externally provided ssh key not only with two
  keys generated by the kernel-fitimage.bbclass itself.
* Add a test which signs only the configuration but not the image nodes.
  There was no test case which covered the probably much more important
  use case of setting FIT_SIGN_INDIVIDUAL = "0".
* Cover also the unbundled initramfs use case. Also this use case is
  probably much more relevant than the bundled initramnfs use case.

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