kbuild: fix generic asm-offsets.h support

iThis fixes a bug where the generated asm-offsets.h file was saved in
the source tree even with make O=.
Thanks to Stephen Rothwell <sfr@canb.auug.org.au> for the report.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/Makefile b/Makefile
index 524fb48..382298f 100644
--- a/Makefile
+++ b/Makefile
@@ -814,7 +814,7 @@
 endif
 
 prepare0: prepare prepare1 FORCE
-	$(Q)$(MAKE) $(build)=$(srctree)
+	$(Q)$(MAKE) $(build)=.
 
 # All the preparing..
 prepare-all: prepare0