kbuild: implement CONFIG_HEADERS_INSTALL for Usermode Linux

userprogs sometimes need access to UAPI headers.
This is currently not possible for Usermode Linux, as UM is only
a pseudo architecture built on top of a regular architecture and does
not have its own UAPI.
Instead use the UAPI headers from the underlying regular architecture.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Author: t-8ch

Makefile

@@ -1361,9 +1361,12 @@ hdr-inst := -f $(srctree)/scripts/Makefile.headersinst obj
 
 PHONY += headers
 headers: $(version_h) scripts_unifdef uapi-asm-generic archheaders archscripts
-	$(if $(filter um, $(SRCARCH)), $(error Headers not exportable for UML))
+ifdef HEADER_ARCH
+	$(Q)$(MAKE) -f $(srctree)/Makefile HEADER_ARCH= SRCARCH=$(HEADER_ARCH) headers
+else
 	$(Q)$(MAKE) $(hdr-inst)=include/uapi
 	$(Q)$(MAKE) $(hdr-inst)=arch/$(SRCARCH)/include/uapi
+endif
 
 ifdef CONFIG_HEADERS_INSTALL
 prepare: headers

lib/Kconfig.debug

@@ -473,7 +473,6 @@ config READABLE_ASM
 
 config HEADERS_INSTALL
 	bool "Install uapi headers to usr/include"
-	depends on !UML
 	help
 	  This option will install uapi headers (headers exported to user-space)
 	  into the usr/include directory for use during the kernel build.