ath79: disable building future NAND images for GL-AR300M/GL-AR750S

ath79: disable building future NAND images for GL-AR300M/GL-AR750S

To simplify the upgrade process and ensure easier identification of
device partitioning, the following devices are disabled on ath79
target in openwrt-19.07 branch:

- glinet,gl-ar300m-nor
- glinet,gl-ar300m-nand
- glinet,gl-ar750s

Proper ath79 (NAND) support for the devices is expected to be
introduced based on kernel 4.19 (see GitHub PR #2184).

In openwrt-19.07, ar71xx should be used for those devices.

With this, we ensure that the new ath79 image names (at least for
releases) refer to the updated partitioning.

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