board/boundarydevices: update u-boot scripts for 3.14 kernels

Those scripts will support coming platforms such as iMX6SoloX or
iMX6QuadPlus.

One important change is that the bootscript now relies on U-Boot to
make the display detection in order to simplify the script.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>