kbuild: make wrapper Makefile more convenient for external modules

When Kbuild starts building in a separate output directory, it generates
a wrapper Makefile, allowing you to invoke 'make' from the output
directory.

This commit makes it more convenient, so you can invoke 'make' without
M= or MO=.

First, you need to build external modules in a separate directory:

  $ make M=/path/to/module/source/dir MO=/path/to/module/build/dir

Once the wrapper Makefile is generated in /path/to/module/build/dir,
you can proceed as follows:

  $ cd /path/to/module/build/dir
  $ make

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

Author: masahir0y

Makefile

@@ -642,10 +642,20 @@ ifdef building_out_of_srctree
 # At the same time when output Makefile generated, generate .gitignore to
 # ignore whole output directory
 
+ifdef KBUILD_EXTMOD
+print_env_for_makefile = \
+	echo "export KBUILD_OUTPUT = $(objtree)"; \
+	echo "export KBUILD_EXTMOD = $(realpath $(srcroot))" ; \
+	echo "export KBUILD_EXTMOD_OUTPUT = $(CURDIR)"
+else
+print_env_for_makefile = \
+	echo "export KBUILD_OUTPUT = $(CURDIR)"
+endif
+
 quiet_cmd_makefile = GEN     Makefile
       cmd_makefile = { \
 	echo "\# Automatically generated by $(abs_srctree)/Makefile: don't edit"; \
-	echo "export KBUILD_OUTPUT = $(CURDIR)"; \
+	$(print_env_for_makefile); \
 	echo "include $(abs_srctree)/Makefile"; \
 	} > Makefile