mpc85xx: disable bricking TL-WDR4900v1 images

mpc85xx: disable bricking TL-WDR4900v1 images

The current mpc85xx build is failing because the
TL-WDR4900v1 kernel image no longer fits into the
partition. Extending the kernel is not possible
without updating u-boot's kernel loader commands.

This patch disables the WDR4900v1 until the kernel
image size issue is fixed so the buildbot can still
compile the Sophos RED 15w Rev.1 . Installing the
WDR4900v1 images would cause the routers to get
bricked.

For the discussion, please go to:
<https://github.com/openwrt/openwrt/pull/1773>

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>