devtool: Load plugins in a well defined order

devtool: Load plugins in a well defined order

To allow devtool plugins in one layer to shadow another in a well
defined way, first search BBPATH/lib/devtool directories and then
scripts/lib/devool 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>