Makefile: remove warnings on initial version check

Android.mk is using a SUBMAKE variable which is only
used to get busybox version as a CFLAG. This one has
the -s (--silent) makeflag. So ignore the KBUILD_OUTPUT
default value if this flag is present.

Change-Id: Ifcc11d148d6707a8c301b3d2ba0370b91215647e