Disable and configure USB Serial Gadget from bb-boot

[jmcintyre%df...@gtempaccount.com]

Would there be any interest in patches to am335x_evm.sh to allow for the configuration of USB Serial Gadget from bb-boot? I have a need to disable the use of getty with /dev/ttyGS0 so that I can create my own serial application, but I also think that it would make sense to add the ability to disable serial gadget entirely just as you can ethernet and mass storage.