kbuild: Do not remove vmlinux when cleaning external module

Since commit 1f2bfbd0 "kbuild:
link of vmlinux moved to a script" make clean with M=<dir>
argument (so cleaning external module) removes vmlinux,
System.map and couple of other files from the *main* kernel
build directory! This not what was happening before and almost
certainly not what one would expect.

This patch moves makes the clean target of the script called
only when !KBUILD_EXTMOD.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Cc: stable@vger.kernel.org [v3.5+]
Signed-off-by: Michal Marek <mmarek@suse.cz>