Update Qt DC Boundary Devices docs

Suggest adding the following in the QtDC Preparing Hardware section for Boundary Devices boards (https://doc.qt.io/QtForDeviceCreation/qtee-preparing-hardware-imx6.html)

Before the section "Setting up USB Access" add:

Setting up Display Resolution

The uboot on the board will attempt to guess which display is connected, sometimes it doesn't get this right. With the uboot version > v2015.07 it is possible to set a uboot environment variable to select the appropriate display resolution for your connected display. Please refer to the Boundary Devices documentation for details https://boundarydevices.com/display-configuration-u-boot/.

...just discovered there is a section on setting the display here https://doc.qt.io/QtForDeviceCreation/qtee-customization.html. Looking at that page, my take is that anything that is generic stays on the customization section. Anything that is board specific should be in the board setup page.