uboot-phytec

vybrid: Define the imx_get_mac_from_fuse() as a __weak function

The proposed way of reading fused MAC in the imx_get_mac_from_fuse() may
be different for other boards.

This commit defines the imx_get_mac_from_fuse() as a weak function to allow
board file overriding it with customized function.

Signed-off-by: Lukasz Majewski <lukma@denx.de>