Introduce new eth_receive routine

The purpose of this routine is receiving a single network frame, outside of
U-Boot's NetLoop(). Exporting it to standalone programs that run on top of
U-Boot will let them utilise networking facilities. For sending a raw frame
the already existing eth_send() can be used.

The direct consumer of this routine is the newly introduced API layer for
external applications (enabled with CONFIG_API).

Signed-off-by: Rafal Jaworowski <raj@semihalf.com>
Signed-off-by: Piotr Kruszynski <ppk@semihalf.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>