recipetool: create: set up priority system for recipe handlers

recipetool: create: set up priority system for recipe handlers

Sometimes we want to force one handler to run before another; if the two
handlers are in different plugins that's difficult without some kind of
priority number, so add one and sort by it.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>