ramips: remove default IMAGE_SIZE for ramips target

ramips: remove default IMAGE_SIZE for ramips target

Currently, ramips target defines 0x7b0000 as default IMAGE_SIZE
for all devices in ramips target, i.e. this will be set if a
device does not specify IMAGE_SIZE itself.

From 92 devices using that default due to a "missing" IMAGE_SIZE,
14 were incorrect by a small amount (i.e. still "8M" flash) and
12 were completely off ("16M", "4M", ...).

This patch thus removes the _default_ IMAGE_SIZE and defines
IMAGE_SIZE for each device individually. This should indicate to
people supporting new devices that this parameter has to be cared
about.

For the present code, this patch is cosmetical.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>