ethernet: Add driver initialization priority to Kconfig

The default kernel init priority is too low. Make this
configurable and set the default priority so that the
ethernet driver is started just before the network stack.

This commit adds generic ethernet priority and changes
currently available ethernet drivers to use it.

Change-Id: If695e52b6dd9ea227f10ba306bb145d72d2312b0
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>