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>

Makefile

@@ -767,7 +767,7 @@ $(vmlinux-dirs): prepare-all scripts
 prepare2:
 ifneq ($(KBUILD_SRC),)
 	@echo '  Using $(srctree) as source for kernel'
-	$(Q)if [ -h $(srctree)/include/asm -o -f $(srctree)/.config ]; then \
+	$(Q)if [ -f $(srctree)/.config ]; then \
 		echo "  $(srctree) is not clean, please run 'make mrproper'";\
 		echo "  in the '$(srctree)' directory.";\
 		/bin/false; \
@@ -779,7 +779,8 @@ endif
 # prepare1 creates a makefile if using a separate output directory
 prepare1: prepare2 outputmakefile
 
-prepare0: prepare1 include/linux/version.h include/asm include/config/MARKER
+prepare0: prepare1 include/linux/version.h $(objtree)/include/asm \
+                   include/config/MARKER
 ifneq ($(KBUILD_MODULES),)
 	$(Q)rm -rf $(MODVERDIR)
 	$(Q)mkdir -p $(MODVERDIR)
@@ -818,7 +819,7 @@ export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)
 #	hard to detect, but I suppose "make mrproper" is a good idea
 #	before switching between archs anyway.
 
-include/asm:
+$(objtree)/include/asm:
 	@echo '  SYMLINK $@ -> include/asm-$(ARCH)'
 	$(Q)if [ ! -d include ]; then mkdir -p include; fi;
 	@ln -fsn asm-$(ARCH) $@