board/boundarydevices: add mac parameter for BD-SDMAC in bootscript

Our new WiFi 802.11ac BT4.1 module (BD-SDMAC [1]) driver allows to
override the wlan MAC address using a module parameter.

Since its driver is now included in our external repository [2], update
the bootscript so it sets the parameter for that driver too.

As a FYI, $wlmac is based on Ethernet MAC address located in fuses.

[1] https://boundarydevices.com/product/bd_sdmac_wifi/
[2] https://github.com/boundarydevices/buildroot-external-boundary



Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>