kbuild: derive relative path for KBUILD_SRC from CURDIR

This enables relocating source and build trees to different roots,
provided they stay reachable relative to one another.  Useful for
builds done within a sandbox where the eventual root is prefixed
by some undesirable path component.

Makefile

@@ -147,7 +147,8 @@ $(filter-out _all sub-make $(CURDIR)/Makefile, $(MAKECMDGOALS)) _all: sub-make
 	@:
 
 sub-make:
-	$(Q)$(MAKE) -C $(KBUILD_OUTPUT) KBUILD_SRC=$(CURDIR) \
+	$(Q)$(MAKE) -C $(KBUILD_OUTPUT) \
+	KBUILD_SRC=$(shell realpath --relative-to=$(KBUILD_OUTPUT) $(CURDIR)) \
 	-f $(CURDIR)/Makefile $(filter-out _all sub-make,$(MAKECMDGOALS))
 
 # Leave processing to above invocation of make