Enable IPv6 support in the iPXE image

Also adds support for making this and any future iPXE build config changes.

Change-Id: Ice0eb5e10e60b1b85e36f695117c1af07e958d6d

ipxe/Makefile

@@ -1,21 +1,55 @@
-IPXE_IMG=ipxe/src/bin/ipxe.iso
+# The location of the directory containing the iPXE make file
+# Override this to build from a different source directory
+IPXE_SRCDIR=ipxe/src
+
+# The specific image to use as the OVB iPXE boot image
+# Override this to use a different image (e.g. ipxe.hd)
+IPXE_IMG=ipxe.iso
+
+ipxe_img_path=$(IPXE_SRCDIR)/bin/$(IPXE_IMG)
 
 all: ipxe-boot.img ipxe-boot.qcow2
 
-ipxe-boot.img: $(IPXE_IMG) script.ipxe
-	cp $(IPXE_IMG) $@
+ipxe-boot.img: $(ipxe_img_path) script.ipxe
+	cp $< $@
 
 ipxe-boot.qcow2: ipxe-boot.img
 	qemu-img convert -f raw -O qcow2 $< $@
 
+# iPXE is configured by setting config macros in the source tree. The repo
+# contains a number of config headers in ipxe/src/config/*.h which contain
+# defaults. These defaults can be overridden by creating a corresponding header
+# in ipxe/src/config/local.
+# For example, the source repo contains ipxe/src/config/general.h, which
+# explicitly does not define NET_PROTO_IPV6. To enable IPv6 support in iPXE we
+# need to create ipxe/src/config/local/general.h and define NET_PROTO_IPV6 in
+# that header.
+# The following allows OVB to keep iPXE configuration under ipxe-config in this
+# repo, and copies it into place in the iPXE repo during build.
+
+# config_headers is a list of the filenames of all overridden headers in ipxe-config/
+config_headers = $(foreach header,$(wildcard ipxe-config/*.h),\
+		           $(patsubst ipxe-config/%,%,$(header)))
+
+# repo_config_path is the path to local override headers in the ipxe repo
+repo_config_path = $(IPXE_SRCDIR)/config/local
+
+# repo_config_headers is a list of all overridden headers in the iPXE repo
+repo_config_headers = $(foreach header,$(config_headers),$(repo_config_path)/$(header))
+
+# Copy individual repo_config_headers from ipxe-config/
+$(repo_config_path): ; mkdir $@
+$(repo_config_path)/%.h: ipxe-config/%.h | $(repo_config_path)
+	cp $< $@
+
 # We disable -Werror so we can build older commits with newer gcc
 # Don't use parallel make, as this races to initialise config headers in a
 # clean repo.
-$(IPXE_IMG):
-	$(MAKE) -C ipxe/src NO_WERROR=1 EMBED=../../script.ipxe bin/ipxe.iso
+$(ipxe_img_path): $(repo_config_headers)
+	$(MAKE) -C ipxe/src NO_WERROR=1 EMBED=../../script.ipxe bin/$(IPXE_IMG)
 
 clean:
 	$(MAKE) -C ipxe/src clean
 	rm -f ipxe-boot.img ipxe-boot.qcow2
 
-.PHONY: $(IPXE_IMG) clean all
+.PHONY: $(ipxe_img_path) clean

ipxe/ipxe-config/README

@@ -0,0 +1,3 @@
+Headers in this directory will be copied to src/config/local/ in the ipxe build
+directory. Values defined in a local header will override the default in the
+corresponding header in src/config/.

ipxe/ipxe-config/general.h

@@ -0,0 +1 @@
+#define	NET_PROTO_IPV6		/* Enable IPv6 */