This renames MICROPY_HW_HAS_WLAN to MICROPY_HW_ENABLE_CC3K (since it's a
driver, not a board feature) and wraps all CC3000 code in this #if.
It's disabled for all boards.

It's not enabled by default because it doesn't fully work.  It can
connect to an AP, get an IP address and do a host-lookup, but not yet do
send or recv on a socket.