eth: add eth tether for wifi AP

Both sam9g25 and sama5d3 take eth as default

Signed-off-by: Royce Jiang <jiangxd@embedinfo.com>
sdimage: sam9g25: add sdimage support for sam9g25ek

Signed-off-by: Royce Jiang <jiangxd@embedinfo.com>
SDcard: support sdcard mount for sam9g25 and sama5d3

The node for the 2 boards are different, we make them separated to
support both of them

Signed-off-by: Royce Jiang <jiangxd@embedinfo.com>