board: siemens: extend factoryset reading for giedi and deneb boards

giedi and deneb are i.MX8X based and have additional data with
WLAN MAC in factoryset container. Enable building for these
boards and adapt factoryset functions to set WLAN MAC and dtb
name in environment.

Signed-off-by: Anatolij Gustschin <agust@denx.de>