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:
2ef117fe4f 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>

include/configs/nitrogen6x.h

@@ -136,6 +136,7 @@
 	"fdt_addr=0x18000000\0" \
 	"boot_fdt=try\0" \
 	"ip_dyn=yes\0" \
+	"usb_pgood_delay=2000\0" \
 	"mmcdevs=0 1\0" \
 	"mmcpart=1\0" \
 	"mmcroot=/dev/mmcblk0p2 rootwait rw\0" \
@@ -205,6 +206,7 @@
 #define CONFIG_EXTRA_ENV_SETTINGS \
 	"bootdevs=" CONFIG_DRIVE_TYPES "\0" \
 	"umsdevs=" CONFIG_UMSDEVS "\0" \
+	"usb_pgood_delay=2000\0" \
 	"console=ttymxc1\0" \
 	"clearenv=if sf probe || sf probe || sf probe 1 ; then " \
 		"sf erase 0xc0000 0x2000 && " \