esp6: Reorganize esp_output

We need a fallback for ESP at layer 2, so split esp6_output
into generic functions that can be used at layer 3 and layer 2
and use them in esp_output. We also add esp6_xmit which is
used for the layer 2 fallback.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>