kbuild: check arch/$(SRCARCH)/include/generated before out-of-tree build

After cross-compiling the kernel, "make mrproper" should be executed
with the proper ARCH= option. Otherwise, stale objects will remain
under arch/$(SRCARCH)/.

One bad scenario is like this:

  $ make ARCH=arm defconfig all   # cross-compile the kernel for arm
  $ make mrproper                 # mrproper for host-arch (i.e. x86)
  $ make ARCH=arm O=build_dir defconfig all

If you miss ARCH= for mrproper and cross-compile the kernel with O=
and ARCH= options, Kbuild will happily start to build, but may fail
due to stale objects in the srctree.

If $(srctree)/arch/$(SRCARCH)/include/generated/ exists, let's stop
the out-of-tree build. To detect this, mrproper should clean only
arch/$(SRCARCH)/include/generated/.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1091,7 +1091,9 @@ PHONY += prepare archprepare prepare1 prepare3
 prepare3: include/config/kernel.release
 ifneq ($(srctree),.)
 	@$(kecho) '  Using $(srctree) as source for kernel'
-	$(Q)if [ -f $(srctree)/.config -o -d $(srctree)/include/config ]; then \
+	$(Q)if [ -f $(srctree)/.config -o \
+		 -d $(srctree)/include/config -o \
+		 -d $(srctree)/arch/$(SRCARCH)/include/generated ]; then \
 		echo >&2 "  $(srctree) is not clean, please run 'make mrproper'"; \
 		echo >&2 "  in the '$(srctree)' directory.";\
 		/bin/false; \
@@ -1350,7 +1352,7 @@ CLEAN_DIRS  += $(MODVERDIR) include/ksym
 
 # Directories & files removed with 'make mrproper'
 MRPROPER_DIRS  += include/config usr/include include/generated          \
-		  arch/*/include/generated .tmp_objdiff
+		  arch/$(SRCARCH)/include/generated .tmp_objdiff
 MRPROPER_FILES += .config .config.old .version \
 		  Module.symvers tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS \
 		  signing_key.pem signing_key.priv signing_key.x509	\