imx: spl: Allow iMX7/8/8M to overwrite spl_board_boot_device

Move the default mapping of spl_boot_device to weak function of
spl_board_boot_device. So that every board of iMX7/8/8M can overwrite
this function to implement specific mapping.

Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
1 file changed