ramips: disable default build for HooToo HT-TM02

ramips: disable default build for HooToo HT-TM02

While the latest version of 19.07 release is usable,
the current master is unbootable on the device in a normal way.

"Normal way" installations includes:
- sysupgrade (e.g. from 19.07)
- RESET button recovery with Ron Curry's (Wingspinner) UBoot image
  (10.10.10.3 + "Kernal.bin")
- RESET button recovery with original U-Boot
  (10.10.10.254 + "kernel")

One could flash and boot the latest master sysupgrade image successfully
with serial access to the device. But a sysupgrade from this state still
breaks the U-Boot and soft-bricks the device.

Signed-off-by: Szabolcs Hubai <szab.hu@gmail.com>