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>

Kbuild

@@ -22,6 +22,7 @@ sed-$(CONFIG_MIPS) := "/^@@@/s///p"
 
 quiet_cmd_offsets = GEN     $@
 define cmd_offsets
+	mkdir -p $(dir $@); \
 	cat $< | \
 	(set -e; \
 	 echo "#ifndef __ASM_OFFSETS_H__"; \
@@ -43,6 +44,6 @@ arch/$(ARCH)/kernel/asm-offsets.s: arch/$(ARCH)/kernel/asm-offsets.c FORCE
 	$(Q)mkdir -p $(dir $@)
 	$(call if_changed_dep,cc_s_c)
 
-$(srctree)/$(offsets-file): arch/$(ARCH)/kernel/asm-offsets.s Kbuild
+$(obj)/$(offsets-file): arch/$(ARCH)/kernel/asm-offsets.s Kbuild
 	$(call cmd,offsets)
 

Makefile

@@ -814,7 +814,7 @@ ifneq ($(KBUILD_MODULES),)
 endif
 
 prepare0: prepare prepare1 FORCE
-	$(Q)$(MAKE) $(build)=$(srctree)
+	$(Q)$(MAKE) $(build)=.
 
 # All the preparing..
 prepare-all: prepare0