imx6: use device-tree compatible for board name

imx6: use device-tree compatible for board name

In imx6, we currently use the model from DTS to derive a board name
manually in /lib/imx6.sh.

However, if we have individual DTS files anyway, we can exploit
generic 02_sysinfo and use the compatible as board name directly.

While at it, remove the wildcards from /lib/upgrade/platform.sh as
these might make code shorter, but are quite unpleasant when grepping
for a specific device.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>