Add an example setting up an env for the previous commit

Quoting:

  As for what ${args} would need to be for this to work:

  - You need to mount a +x build system prep script to /input.sh

  - You need to mount a +x build system output extractor to /output.sh

  - You need to mount a folder with input resources to /input

  - You need to mount an output folder to /output

  So, /input.sh could `cp /input/diffconfig /builder/openwrt/.config`,
  then cd to /builder/openwrt and run `make defconfig`. The ${CMD}
  could then actually build, and /output.sh could collect logs and
  build artefacts to /output.

This lot provides exactly that. The makefile in this new dir just
requires diffconfig to be populated; then run `make output` and wait.

Q: What about adding LibreMesh

A: Just add to input.sh (or add a script to it) which does the steps
described here: https://libremesh.org/development.html. You can avoid
the 'make menuconfig' bits by pre-building a diffconfig as needed. And
if you need to debug, add a /bin/sh.

openwrt/Makefile

@@ -0,0 +1,31 @@
+define INC_NEWOPENWRT =
+ 1
+endef
+$(info Defined INC_NEWOPENWRT)
+
+include /devel/docker/docker_vars.sh
+export
+
+unexport INC_DOCKER
+ifndef INC_DOCKER
+include ${docker_src}/Makefile
+endif
+
+.PHONY: input
+input:
+	rm -rf input; mkdir -p input; chmod 777 input;
+
+.PHONY: prep-output
+prep-output:
+	rm -rf output; mkdir output; chmod 777 output;
+
+input/diffconfig: diffconfig input
+	cp $< $@
+
+output: diffconfig ${docker_src}/built/openwrt-builder-master.sentinel prep-output input/diffconfig input.sh output.sh
+	docker run \
+		-v $$PWD/output:/output \
+		-v $$PWD/input:/input \
+		-v $$PWD/input.sh:/input.sh \
+		-v $$PWD/output.sh:/output.sh \
+		--rm -it openwrt-builder-master make V=ssc -j24 build

openwrt/input.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+cd /builder/openwrt
+
+cp /input/diffconfig .config
+
+make defconfig

openwrt/output.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+
+cd /builder/openwrt
+
+cp -r bin /output/
+
+cp .config /output/
+./scripts/diffconfig.sh > diffconfig
+cp diffconfig /output/