kbuild: refactor headers_* targets in Makefile

o Use lower case for local variables
o Add a helper target for common targets
o Use $(hdr-inst)= ... to make Make invocations simpler
o Add -rR to make invocations

In total this adds more lines than it removes but the
benefit is better readability
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: David Woodhouse <dwmw2@infradead.org>

Makefile

@@ -1010,36 +1010,43 @@ firmware_install: FORCE
 
 # ---------------------------------------------------------------------------
 # Kernel headers
-INSTALL_HDR_PATH=$(objtree)/usr
-export INSTALL_HDR_PATH
 
-HDRFILTER=generic i386 x86_64
-HDRARCHES=$(filter-out $(HDRFILTER),$(patsubst $(srctree)/include/asm-%/Kbuild,%,$(wildcard $(srctree)/include/asm-*/Kbuild)))
+#Default location for installed headers
+export INSTALL_HDR_PATH = $(objtree)/usr
 
-PHONY += headers_install_all
-headers_install_all: include/linux/version.h scripts_basic FORCE
+hdr-filter := generic um ppc
+hdr-archs  := $(filter-out $(hdr-filter),                           \
+                  $(patsubst $(srctree)/include/asm-%/Kbuild,%,     \
+                      $(wildcard $(srctree)/include/asm-*/Kbuild)))
+hdr-inst := -rR -f $(srctree)/scripts/Makefile.headersinst obj
+
+PHONY += __headers
+__headers: include/linux/version.h scripts_basic FORCE
 	$(Q)$(MAKE) $(build)=scripts scripts/unifdef
-	$(Q)for arch in $(HDRARCHES); do \
-	 $(MAKE) ARCH=$$arch -f $(srctree)/scripts/Makefile.headersinst obj=include BIASMDIR=-bi-$$arch ;\
+
+PHONY += headers_install_all
+headers_install_all: __headers
+	$(Q)for arch in $(hdr-archs); do \
+	 $(MAKE) ARCH=$$arch $(hdr-inst)=include BIASMDIR=-bi-$$arch ;\
 	 done
 
 PHONY += headers_install
-headers_install: include/linux/version.h scripts_basic FORCE
-	@if [ ! -r $(srctree)/include/asm-$(SRCARCH)/Kbuild ]; then \
+headers_install: __headers
+	$(Q)if [ ! -r $(srctree)/include/asm-$(SRCARCH)/Kbuild ]; then \
 	  echo '*** Error: Headers not exportable for this architecture ($(SRCARCH))'; \
-	  exit 1 ; fi
-	$(Q)$(MAKE) $(build)=scripts scripts/unifdef
-	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.headersinst ARCH=$(SRCARCH) obj=include
+	  exit 1 ; \
+	fi
+	$(Q)$(MAKE) $(hdr-inst)=include ARCH=$(SRCARCH)
 
 PHONY += headers_check_all
 headers_check_all: headers_install_all
-	$(Q)for arch in $(HDRARCHES); do \
-	 $(MAKE) ARCH=$$arch -f $(srctree)/scripts/Makefile.headersinst obj=include BIASMDIR=-bi-$$arch HDRCHECK=1 ;\
+	$(Q)for arch in $(hdr-archs); do \
+	 $(MAKE) ARCH=$$arch $(hdr-inst)=include BIASMDIR=-bi-$$arch HDRCHECK=1 ;\
 	 done
 
 PHONY += headers_check
 headers_check: headers_install
-	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.headersinst ARCH=$(SRCARCH) obj=include HDRCHECK=1
+	$(Q)$(MAKE) $(hdr-inst)=include ARCH=$(SRCARCH) HDRCHECK=1
 
 # ---------------------------------------------------------------------------
 # Modules