kbuild: asm-generic support

There is an increasing amount of header files
shared between individual architectures in asm-generic.
To avoid a lot of dummy wrapper files that just
include the corresponding file in asm-generic provide
some basic support in kbuild for this.

With the following patch an architecture can maintain
a list of files in the file arch/$(ARCH)/include/asm/Kbuild

To use a generic file just add:

        generic-y += <name-of-header-file.h>

For each file listed kbuild will generate the necessary
wrapper in arch/$(ARCH)/include/generated/asm.

When installing userspace headers a wrapper is likewise created.

The original inspiration for this came from the unicore32
patchset - although a different method is used.

The patch includes several improvements from Arnd Bergmann.
Michael Marek contributed Makefile.asm-generic.

Remis Baima did an intial implementation along to achive
the same - see https://patchwork.kernel.org/patch/13352/

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Guan Xuetao <guanxuetao@mprc.pku.edu.cn>
Tested-by: Guan Xuetao <guanxuetao@mprc.pku.edu.cn>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: Remis Lima Baima <remis.developer@googlemail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Makefile

@@ -349,7 +349,8 @@ CFLAGS_GCOV	= -fprofile-arcs -ftest-coverage
 
 # Use LINUXINCLUDE when you must reference the include/ directory.
 # Needed to be compatible with the O= option
-LINUXINCLUDE    := -I$(srctree)/arch/$(hdr-arch)/include -Iinclude \
+LINUXINCLUDE    := -I$(srctree)/arch/$(hdr-arch)/include \
+                   -Iarch/$(hdr-arch)/include/generated -Iinclude \
                    $(if $(KBUILD_SRC), -I$(srctree)/include) \
                    -include include/generated/autoconf.h
 
@@ -417,6 +418,12 @@ ifneq ($(KBUILD_SRC),)
 	    $(srctree) $(objtree) $(VERSION) $(PATCHLEVEL)
 endif
 
+# Support for using generic headers in asm-generic
+PHONY += asm-generic
+asm-generic:
+	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.asm-generic \
+	            obj=arch/$(SRCARCH)/include/generated/asm
+
 # To make sure we do not include .config for any of the *config targets
 # catch them early, and hand them over to scripts/kconfig/Makefile
 # It is allowed to specify more targets when calling make, including
@@ -954,7 +961,7 @@ ifneq ($(KBUILD_SRC),)
 endif
 
 # prepare2 creates a makefile if using a separate output directory
-prepare2: prepare3 outputmakefile
+prepare2: prepare3 outputmakefile asm-generic
 
 prepare1: prepare2 include/linux/version.h include/generated/utsrelease.h \
                    include/config/auto.conf
@@ -1028,7 +1035,7 @@ hdr-inst := -rR -f $(srctree)/scripts/Makefile.headersinst obj
 hdr-dst = $(if $(KBUILD_HEADERS), dst=include/asm-$(hdr-arch), dst=include/asm)
 
 PHONY += __headers
-__headers: include/linux/version.h scripts_basic FORCE
+__headers: include/linux/version.h scripts_basic asm-generic FORCE
 	$(Q)$(MAKE) $(build)=scripts build_unifdef
 
 PHONY += headers_install_all
@@ -1143,7 +1150,8 @@ CLEAN_FILES +=	vmlinux System.map \
                 .tmp_kallsyms* .tmp_version .tmp_vmlinux* .tmp_System.map
 
 # Directories & files removed with 'make mrproper'
-MRPROPER_DIRS  += include/config usr/include include/generated
+MRPROPER_DIRS  += include/config usr/include include/generated          \
+                  arch/*/include/generated
 MRPROPER_FILES += .config .config.old .version .old_version             \
                   include/linux/version.h                               \
 		  Module.symvers tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS