kbuild: support include/generated

We need a location for generated files.
Today they are spread over several places and bringing them
together to a common place makes it obvious hat is generated
and what isreal files.

Al Viro originally suggested: include/gen
Linus suggested to spell it out.

This patch implement support for

    include/generated

All files in include/generated are ignored by git.
include/generated is removed during "make mrproper".

With this we are ready to implement support for include/generated
in the various architctures and in the base kernel.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>

.gitignore

@@ -49,6 +49,7 @@ include/linux/compile.h
 include/linux/version.h
 include/linux/utsrelease.h
 include/linux/bounds.h
+include/generated
 
 # stgit generated dirs
 patches-*

Makefile

@@ -1200,7 +1200,7 @@ CLEAN_FILES +=	vmlinux System.map \
                 .tmp_kallsyms* .tmp_version .tmp_vmlinux* .tmp_System.map
 
 # Directories & files removed with 'make mrproper'
-MRPROPER_DIRS  += include/config include2 usr/include
+MRPROPER_DIRS  += include/config include2 usr/include include/generated
 MRPROPER_FILES += .config .config.old include/asm .version .old_version \
                   include/linux/autoconf.h include/linux/version.h      \
                   include/linux/utsrelease.h                            \