configs: add defconfig for TS-4900

configs: add defconfig for TS-4900

The TS-4900 uses a custom Linux 4.X from Technologic Systems:
https://github.com/embeddedarm/linux-3.10.17-imx6/tree/imx_4.1.15_1.0.0_ga
Indeed, it is not supported by mainline Linux but this work is ongoing.

This patch is inspired by a previous work for the TS-4800.
A post image script is provided to generate an image that can be
directly written to an SD card. It also adds an fpga wifi module.

Signed-off-by: Mathieu Audat <mathieu.audat@savoirfairelinux.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>