cc.h: Disable Ethernet frame padding

cc.h: Disable Ethernet frame padding

We set ETH_PAD_SIZE to 0 and remove support from uknetdev.c. This is done
because some uknetdev drivers will not support it (e.g., netfront) and
according to https://lwn.net/Articles/89597/, the gains of doing padding
is negligible on the CPU architectures that we currently support while
potentially hurting DMA engine performances that may get involved with
the respective driver and/or hypervisor.

Signed-off-by: Simon Kuenzer <simon.kuenzer@neclab.eu>
Reviewed-by: Sharan Santhanam <sharan.santhanam@neclab.eu>