ipq40xx: meraki-mr33, meraki-mr74: disable image generation

ipq40xx: meraki-mr33, meraki-mr74: disable image generation

After migrating to kernel 5.15, upgrading causes the units to become
soft-bricked, hanging forever at the kernel startup.
Kernel size limitation of 4000000 bytes is suspected here, but this is
not fully confirmed.

Disable the images to protect users from inadvertent bricking of units,
because recovery of those is painful with Cisco's U-boot, until the root
cause is found and fixed.

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>