FR: `upload_port = usb`

[pfeerick]

As a follow on to https://github.com/platformio/platform-atmelavr/issues/173, a feature request:

Can upload_port take usb as an option, so that instead of setting upload_flags = -Pusb to set the port, upload_flags = usb can be used?

It looks like it would fit in after the test for custom as another state: https://github.com/platformio/platform-atmelavr/blob/0c0972b8e5216cc4e87b4292dbe4fb66d58c8a83/builder/main.py#L187

[theamk]

See comment in #173, the workaround is to add this line to platformio.ini: board_upload.require_upload_port = False

(but this should really be default per upload_protocol)