toaster: Add new ReST API for Image Customisation feature

toaster: Add new ReST API for Image Customisation feature

Implemented xhr_customrecipe API. To create a custom recipe from a
base recipe.
Implemented xhr_customrecipe_packages API to add/remove packages
to/from custom recipe.

co-authored see Signed-off-by

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Elliot Smith <elliot.smith@intel.com>
Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: brian avery <avery.brian@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>