Enable easy building with the NERF u-root tree #317

This adds a `CONFIG_UROOT=y` option to allow the busybox
runtime to be replaced with the go u-root runtime.
You must have go 1.9 or newer for it to work.

It has been tested on the OCP winterfell and qemu nodes,
and it can be specified on the build command line as well.

Nothing from `heads/initrd` or any of the tools will be
linked into the cpio file.  Only the kernel modules and the
go shell will be included.

modules/u-root

@@ -0,0 +1,33 @@
+#
+# u-root go runtime
+#
+# This is not a normal Heads module, since it builds all of its
+# own dependencies.
+#
+
+ifeq "$(CONFIG_UROOT)" "y"
+
+u-root_url := github.com/u-root/u-root
+u-root_output := $(build)/$(BOARD)/initrd.cpio
+
+export GOPATH=$(build)/go
+
+$(GOPATH):
+	$(call do,GO GET,$(u-root_url),\
+		go get $(u-root_url) \
+	)
+
+$(u-root_output): $(GOPATH)
+	$(call do,U-ROOT,$@,\
+		$(GOPATH)/bin/u-root \
+			-build=bb \
+			-format=cpio \
+			-o $@ \
+	)
+
+# Override the initrd inputs and add in the kernel modules
+INITRD_OVERRIDE += $(u-root_output)
+INITRD_OVERRIDE += $(build)/$(BOARD)/modules.cpio
+
+
+endif