buildkernel now concurrent for most modules on master

[Matthew Dillon]

The buildkernel now runs concurrently on module subdirectories, which
should substantially reduce the build time.

full kernel build times: (make buildkernel -j <ncores>)
on monster (48-core opteron)			10:48 -> 3:04
on brix    (2-core/4-thread mobile broadwell)	 8:30 -> 6:17

Please report any issues with broken dependencies.

					-Matt
					Matthew Dillon 
					<dillon at backplane.com>