mediatek: unset CONFIG_CMDLINE_OVERRIDE for all targets but mt7629

mediatek: unset CONFIG_CMDLINE_OVERRIDE for all targets but mt7629

The newly introduced config symbol CONFIG_CMDLINE_OVERRIDE is only set
for mt7629 for now which breaks automated build on all other mediatek
subtargets. Make sure the symbol is configured as 'is not set' for all
remaining subtargets.

Fixes: c27279dc26 ("mediatek: add support for ipTIME A6004MX Add basic support for ipTIME A6004MX.")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>