build: improve message for incompatible image on "legacy" devices

build: improve message for incompatible image on "legacy" devices

It has been reported that the current message displayed during
upgrade with compat_version change is misleading for "legacy"
devices, i.e. those without the "new" fwtool. This is partially
caused by the fact that we need to exploit the supported_devices
string to get some message text displayed for these devices.

This patch modifies the message to make it more helpful and
include additional information, e.g.

  Device linksys,wrt3200acm not supported by this image
  Supported devices: linksys,wrt3200acm linksys-whateverelse - Image
  version mismatch: image 1.1, device 1.0. Please wipe config during
  upgrade (force required) or reinstall. Reason: Config cannot be
  migrated from swconfig to DSA

Note that the line breaks (except the one before Supported devices)
are added manually here, I hesitate to hack \n into the
supported_devices as well. The "Reason:" will only be displayed if
DEVICE_COMPAT_MESSAGE is set for the device, otherwise
"Please check documentation ..." will be shown instead.

While at it, also rearrange the code in image-commands.mk to
make lines shorter and remove the double filter-out command.

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