quickly build a single program ? (was KERNCONF and KERNFAST related)

how hard would it be to modify the build scripts so that they only try
to rebuild a single program from a tree where you already made a buildworld ?

i usually do modifications to a single program at a time and
it is slightly annoying to wait for the build process to scan
through all the already-built programs to find there is nothing
to do for them.

I guess one would have to override SUBDIR in the main Makefile.inc1,
and then again in the children Makefiles, taking each time the next
component of the filename...

	cheers
	luigi