[hikey][build] netboot option not enabled with -m option in hikey

currently with "-m" option netsvc.netboot is not getting set to true.
Bootimage for hikey is generated using mkbootimg and uses a separate
cmdline option. Fixed by passing the cmdline option to mkbootimg

TEST: Build zircon and flashed on hikey. Enabled init-ums usb option
./scripts/build-zircon-arm64 && ./scripts/flash-hikey -m
usbctl init-ums

Change-Id: I32e9d357591dbc466749ea316a313cd09b9f5dd0