kbuild: Don't fail if include/asm symlink exists

kbuild: Don't fail if include/asm symlink exists

From: Andreas Gruenbacher <agruen@suse.de>

We're having the following situation: There are user-space applications
that include kernel headers directly. With a completely unconfigured
/usr/src/linux tree, including most headers fails because essential
files are not there:

	include/asm
	include/linux/autoconf.h
	include/linux/version.h

So we create these files. On the other hand, we want to use
/usr/src/linux as read-only source for building kernels or additional
modules. Now when building a kernel with a separate output directory
(O=), there is a check in the main makefile for the include/asm symlink.
There is no real need for this check: if we ensure that
$(objdir)/include/asm is always created as the patch does,
$(srctree)/include/asm becomes irrelevant.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>