toaster: orm Add get_base_recipe_file to CustomImageRecipe

toaster: orm Add get_base_recipe_file to CustomImageRecipe

This function returns the base recipe file path only if it currently
exists. This allows us to know whether we can proceed at this point with
generating a custom image recipe. It also enables us to call this
function from the templates to enable visual indication of this state.

Some whitespace fixes also added in generate_recipe_file_contents

Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Elliot Smith <elliot.smith@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>