A lot of things break if `GNRC_NETIF_FLAGS_HAS_L2ADDR` is not set.
In order to handle router advertisements and auto-configureation,
generate a faux l2 address based on the netdev ID.
This changes the prefixes of the symbols generated from USEMODULE and
USEPKG variables. The changes are as follow:
KCONFIG_MODULE_ => KCONFIG_USEMODULE_
KCONFIG_PKG_ => KCONFIG_USEPKG_
MODULE_ => USEMODULE_
PKG_ => USEPKG_
`tsrb` changed to that type, so there is no reason for us, to keep the
internally used buffer as a `char` (especially since the rest of the
driver already uses `uint8_t`)
This simplifies and solidifies the reversal of SLIP's byte-stuffing
(aka byte-unstuffing ;-)) by
1. Using `tsrb` instead of `ringbuffer`: there are two actors. The ISR
and the device's event handler.
2. Moving the byte-unstuffing from the UART RX-handler (i.e. the ISR)
to the device's receive function (potentially not the ISR)
3. Removing the `pktfifo` member. The current number of bytes in the
ringbuffer is returned for `recv(data = NULL, len = 0)`. If that is
more than the packet contains (due to the byte stuffing it most
likely will be) the packet is reallocated in GNRC anyway.
