tools: Define a Makefile function to do subdir processing

Define a Makefile function that can be called with $(call ...) to wrap
the subdir make invocations in tools/Makefile.

This will allow us in the next patch to insert bits in there to honour
O= flags when called from the top-level Makefile.

Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Borislav Petkov <bp@amd64.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1378.1352379110@warthog.procyon.org.uk
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>