`len` and `count` are both values that should never go `< 0`, so instead of
having to test this (in theory) every time the function is called (regardless
of by assert or if its unnecessary code), I propose to change it to `size_t`.
As a bonus I made the type of recv's buf parameter generic - no reason for it to
be a char and it might lead to unnecessary static casting requirements
This patch adds two new options to netopt. NETOPT_CCA_MODE to set
the CCA mode corresponding to the respective PHY standard and
NETOPT_RF_TESTMODE to set the test mode for the radio.
On OS X, `netdev2_tap` suspends monitoring file descriptor until `_recv` is
called. If no spaces in left in pktbuf, `gnrc_netdev2_eth` does not call `_recv`
that results in deadlock.
With this commit, `gnrc_netdev2_eth` calls `_recv` with NULL buffer and non-zero
length parameter, that indicates the driver to drop frame and resume working.
