recipetool: Load plugins in a well defined order

recipetool: Load plugins in a well defined order

To allow recipetool plugins in one layer to shadow another in a well
defined way, first search BBPATH/lib/recipetool directories and then
scripts/lib/recipetool and load only the first found.

The previous search and load loop would load all found plugins with the
ones found later replacing any found before.

Signed-off-by: Ola x Nilsson <ola.x.nilsson@axis.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>