mgcoge make ether_scc.c work with CONFIG_NET_MULTI

This change is needed for mgcoge because it uses two ethernet drivers.

Add a check for the presence of the PIGGY board on mgcoge.  Without this
board networking cannot work and the initialization must be aborted.

Only allocate rtx once to prevent DPRAM exhaustion.

Initialize ether_scc.c and the keymile-specific HDLC driver (to be added
soon) in eth.c.

Signed-off-by: Gary Jennejohn <garyj@denx.de>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>