ar71xx: disable some tiny targets with kernel 4.14

ar71xx: disable some tiny targets with kernel 4.14

The bump to kernel 4.14 caused a massive increase in kernel size.

For most targets, switching them to dynamic partitioning allowed
to cope with this.

On some targets, the kernel partition is located behind the rootfs,
which disallows switching to dynamic partitioning as the boot location
would be altered, requiring a u-boot change.

Also within the tiny section, which disables kernel symbols etc
to decrease the image size,  the partition size is still too small.

Disable these targets for now, fixing image generation:

- Buffalo BHR-4GRV2
- Zbtlink ZBT-WE1526

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>