kbuild: check asm symlink when building a kernel

We often hit the situation where the asm symlink
in include/ points to the wrong architecture.
In 9 out of 10 cases thats because we forgot to set
ARCH but sometimes we just reused the same tree
for another ARCH. For the merged x86 tree we need
to create a new symlink but this is not obvious.
So with the following patch we check if the symlink
points to the correct architecture and error
out if this is not the case.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -903,14 +903,24 @@ prepare: prepare0
 
 export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)
 
-# FIXME: The asm symlink changes when $(ARCH) changes. That's
-# hard to detect, but I suppose "make mrproper" is a good idea
-# before switching between archs anyway.
-
-include/asm:
-	@echo '  SYMLINK $@ -> include/asm-$(SRCARCH)'
-	$(Q)if [ ! -d include ]; then mkdir -p include; fi;
-	@ln -fsn asm-$(SRCARCH) $@
+# The asm symlink changes when $(ARCH) changes.
+# Detect this and ask user to run make mrproper
+
+include/asm: FORCE
+	$(Q)set -e; asmlink=`readlink include/asm | cut -d '-' -f 2`;   \
+	if [ -L include/asm ]; then                                     \
+		if [ "$$asmlink" != "$(SRCARCH)" ]; then                \
+			echo "ERROR: the symlink $@ points to asm-$$asmlink but asm-$(SRCARCH) was expected"; \
+			echo "       set ARCH or save .config and run 'make mrproper' to fix it";             \
+			exit 1;                                         \
+		fi;                                                     \
+	else                                                            \
+		echo '  SYMLINK $@ -> include/asm-$(SRCARCH)';          \
+		if [ ! -d include ]; then                               \
+			mkdir -p include;                               \
+		fi;                                                     \
+		ln -fsn asm-$(SRCARCH) $@;                              \
+	fi
 
 # Generate some files
 # ---------------------------------------------------------------------------