Add serial# for SPL SDP download

Add serial# for SPL SDP download

After update this, uuu(>1.6) can use below command to filter out devices
when multi boards connected.

uuu -ms <serial#> ...
[sudo] uuu -lsusb can list known devices with serial# informaiton.

Serial number is retrieved via get_board_serial() and passed to
g_dnl_set_serialnumber(). This part is conditionally compiled for i.MX94
only now.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Alice Guo <alice.guo@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>