http-file-api: add package for uploading files

[andrenarchy]

As outlined in freifunk-berlin/firmware#484 we need a way to upload (large) files. This PR adds a package with a cgi script that allows file uploads with ubus session authentication+authorization (requires ubus.file.write privileges).

Example curl request:

curl -H 'Authorization: UBUS_SESSION' \
  -H 'Content-Type: application/octet-stream' \
  --data-binary @filename.bin \
  http://x.x.x.x/cgi-bin/files?filename=/tmp/firmware.bin

[booo]

Closing this due to inactivity.