oeqa/runtime/login: Various code improvements and fixes

oeqa/runtime/login: Various code improvements and fixes

* Allow tools to be found from the host PATH so that imagemagick from a buildtools
  tarball/sdk can work
* Reformat the code to have imports at the start of the file and have more standard
  formatting and whitespace
* Always save copies of the images, the space imapct is negligle compared to the
  debug win
* Write the images to ${T}
* Use bb.utils.mkdirhier() instead of more complex code
* Restrict the tests to images containing matchbox-desktop

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>