kbuild: Remove useless warning while appending KCFLAGS

This is a respin of an older patch sent by Sam Ravnborg:
  http://www.gossamer-threads.com/lists/linux/kernel/1530602



This patch removes the annoying warning:

Makefile:708: "WARNING: Appending $KCFLAGS (-Wno-sign-compare) from command
line to kernel $CFLAGS"

which is printed every time I use KCFLAFS. The commit which introduced the
warning:

69ee0b35 kbuild: do not pick up CFLAGS from the environment

tells about the problems when people have CFLAGS in their environment,
then switches to KCFLAFS which should be enough to solve the issue, but
it anyway introduces a warning. Drop this warning.

Signed-off-by: Ozan Çağlayan <ozancag@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>