devtool: return specific exit code for incompatible recipes

devtool: return specific exit code for incompatible recipes

Certain recipes cannot be used with devtool extract / modify / upgrade -
usually because they don't provide any source. Return a specific exit
code (4) so that scripts such as scripts/contrib/devtool-stress.py know
the difference between this and a genuine failure.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>