kbuild: Use relative path when building in a subdir of the source tree

When doing make O=<subdir>, use '..' to refer to the source tree. This
allows for more readable compiler messages, and, more importantly, it
sets the VPATH to '..', so filenames in WARN_ON() etc. will be shorter.
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Makefile

@@ -149,7 +149,17 @@ else
 _all: modules
 endif
 
-srctree		:= $(if $(KBUILD_SRC),$(KBUILD_SRC),.)
+ifeq ($(KBUILD_SRC),)
+        # building in the source tree
+        srctree := .
+else
+        ifeq ($(KBUILD_SRC)/,$(dir $(CURDIR)))
+                # building in a subdirectory of the source tree
+                srctree := ..
+        else
+                srctree := $(KBUILD_SRC)
+        endif
+endif
 objtree		:= .
 src		:= $(srctree)
 obj		:= $(objtree)