kbuild: Inform user to pass ARCH= for make mrproper

When cross-compiling an out-of-tree build with an unclean source tree
directory, the build fails with:

  /path/to/kernel/source/tree is not clean, please run 'make mrproper'
  in the '/path/to/kernel/source/tree' directory.

However, doing so does not fix the problem, as "make mrproper" now
requires passing the target architecture to the make command, else it
won't remove $(srctree)/arch/$(SRCARCH)/include/generated.
"git ls-files -o" doesn't give a clue, as it doesn't list (empty)
directories, only files.

Improve usability by including the ARCH= option in the error output.

Fixes: a788b2ed81 ("kbuild: check arch/$(SRCARCH)/include/generated before out-of-tree build")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1107,7 +1107,7 @@ ifdef building_out_of_srctree
 	$(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 "  $(srctree) is not clean, please run 'make ARCH=$(ARCH) mrproper'"; \
 		echo >&2 "  in the '$(srctree)' directory.";\
 		/bin/false; \
 	fi;