Net: clarify board/cpu_eth_init calls

This has always been confusing, and the idea of these functions returning the
number of interfaces initialized was half-baked and ultimately pointless.
Instead, act more like regular functions and return < 0 on failure, >= 0 on
success.

This change shouldn't break anything.

Signed-off-by: Ben Warren <biggerbadderben@gmail.com>
1 file changed