drivers: ethernet: provide Kconfig means to disable ethernet drivers

There are several test cases that create fake ethernet devices and
expect the fake device to be the only ethernet device enabled.  Some
tests handle this be explicitly disabling actual ethernet drivers,
but this doesn't scale well.

Change drivers/ethernet/Kconfig to utilze a menuconfig option that
wraps all the drivers.  This allows us for those test cases that
don't want any actual ethernet drivers to disable them with a
simple CONFIG_ETH_DRIVER=n.

Note, the fake ethernet devices utilize CONFIG_ETH_INIT_PRIORITY so
we have it outside of the 'if ETH_DRIVER' block.

Signed-off-by: Kumar Gala <galak@kernel.org>