Separate targets into their own repo so they can be forked and linked via github

[jacobrosenthal]

Technically I can fork them, but its onerous to share with others as yotta cant handle digging targets out of subdirectories, and I doubt itll be a priority. IE
this doesn't work yt link-target github.com/ARMmbed/target-nordic-nrf51822

Another important feature lost is its difficult to reference a branch or HEAD which hasnt been pushed to yotta yet.

See ARMmbed/yotta#566

[andresag01]

Hi @jacobrosenthal, thanks for raising this issue. We will address it as soon as we can.

[jacobrosenthal]

Would you guys please PLEASE please just split these apart. Literally this is the worst part of mbed right now. I have patches which I need to generate .dat and .zip which I can't pull request. (and before that fota bootloader which became duplicated work as a result)

Further its increasingly impossible for me keep my forks maintained, updated, tested and working pushing me to file obscure target based bug reports nobody wants:
ARMmbed/ble-examples#53

Please pull these into their own separate repo so I can inherit forks from a target.json.