kbuild: split exported generic header creation into uapi-asm-generic

When we install headers, we are interested only in headers under uapi
directories.  Split out uapi-asm-generic target and make headers_install
depend on it.  It will avoid generating unneeded asm-generic wrappers.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -456,10 +456,11 @@ ifneq ($(KBUILD_SRC),)
 endif
 
 # Support for using generic headers in asm-generic
-PHONY += asm-generic
-asm-generic:
+PHONY += asm-generic uapi-asm-generic
+asm-generic: uapi-asm-generic
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.asm-generic \
 	            src=asm obj=arch/$(SRCARCH)/include/generated/asm
+uapi-asm-generic:
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.asm-generic \
 	            src=uapi/asm obj=arch/$(SRCARCH)/include/generated/uapi/asm
 
@@ -1143,7 +1144,7 @@ PHONY += archscripts
 archscripts:
 
 PHONY += __headers
-__headers: $(version_h) scripts_basic asm-generic archheaders archscripts
+__headers: $(version_h) scripts_basic uapi-asm-generic archheaders archscripts
 	$(Q)$(MAKE) $(build)=scripts build_unifdef
 
 PHONY += headers_install_all