base-files: fwtool: make compat_version backward compatible

So far, the compatibility mechanism only works if both device and
image are already updated to the new routines. This patch extends
the sysupgrade metadata and fwtool_check_image() to account for
"older" images as well:

The basic mechanism for older devices to check for image compatibility
is the supported_devices entry. This can be exploited by putting
a custom message into this variable of the metadata, so older FW
will produce a mismatch and print the message as it thinks it's the
list of supported devices. So, we have two cases:

device 1.0, image 1.0:
  The metadata will just contain supported_devices as before.

device 1.0, image 1.1:
  The metadata will contain:

  "new_supported_devices":["device_string1", "device_string2", ...],
  "supported_devices":["Image version 1.1 incompatible to device: ..."]

  If the device is "legacy", i.e. does not have the updated fwtool.sh,
  it will just fail with image check and print the content of
  supported_devices. If DEVICE_COMPAT_MESSAGE is set, this will be
  printed on old devices as well through the same mechanism. Otherwise
  a generic "Please check documentation ..." is appended.

  Upgrade can still be performed with -F like when
  SUPPORTED_DEVICES has been removed to prevent bricking.

  If the device has updated fwtool.sh (but is 1.0), it will just use
  the new_supported_devices instead, and work as intended (flashing
  with -n will work, flashing without will print the appropriate
  warning).

This mechanism should provide a fair tradeoff between simplicity
and functionality.

Since we touched a lot of fields in metadata, this also bumps
metadata_version to 1.1.

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

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