Document OS_URI option

[jedahan]

Seems pi-maker is relatively generic way of prebuilding arm images for the raspberry pi. In fact, it might be generic to any arm board (which would make it really nice for other SBCs). It would be nice to see the community kinda come together on just a couple of pieces of technology, so showing examples of how to build

• arch64 on pi zero w
• raspbian on pi3b(+)
• armbian on rock64

Would go a long way to encourage coalescence on pi-maker as the host build tool of choice. Maybe just testing that these work and adding them to the readme?

# Examples
## arch64 on pi zero w

    make && make run

## raspbian on pi3b+

    export OS_URI=http://vx2-downloads.raspberrypi.org/raspbian_lite/images/raspbian_lite-2018-06-29/2018-06-27-raspbian-stretch-lite.zip
    make && make run

## armbian on rock64

    export OS_URI=https://dl.armbian.com/rock64/nightly/Armbian_5.51.180704_Rock64_Debian_stretch_default_4.4.138.7z
    make && make run

For reference, prototype-cjdns-pi supports a ton of boards, and I think there's nothing stopping me from using pi-maker with their scripts, but I haven't tried yet.

[westlane]

We purchased a Rock64 and Pi3B+ devices and will run some tests to verify that everything runs smoothly, going beyond the Raspberry Pi Zero W. Thanks for this idea!