imx: nitrogen6x: fix USB host initialization

USB Host scanning has been broken since v2016.05.

This is due to all the USB changes that happened between v2016.03
and v2016.05, especially:
2ef117fe usb: Remove 200 ms delay in usb_hub_port_connect_change()
a22a264ec3 usb: Change power-on / scanning timeout handling

So we need to increase the init delay to 2s using the usb_pgood_delay
environment variable.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>