base-files: fwtool: implement compatibility check for images

We regularly encounter the situation that devices are subject to
changes that will make them incompatible to previous versions.
Removing SUPPORTED_DEVICES will not really be helpful in most of these
cases, as this only helps after a rename.

To solve this situation, this patchset introduces a compatibility
version for devices. In this patch, the actual checks are implemented
into fwtool_check_image():

If an incompatible change is introduced, one can increase either
the minor version (1.0->1.1) or the major version (1.0->2.0).

Minor version increment:
This will still allow sysupgrade, but require to reset config
(-n or SAVE_CONFIG=0). If sysupgrade is called without -n, a
corresponding message will be printed. If sysupgrade is called
with -n, it will just pass, with supported devices being checked
as usual. (Which will allow us to add back SUPPORTED_DEVICES for
many cases.)

Major version increment:
This is meant for potential (rare) cases where sysupgrade is
not possible at all, because it would break the device.
In this case, a warning will be printed, and -n won't help.

If image check fails because of one of the versions parts not
matching, the content of DEVICE_COMPAT_MESSAGE is printed in
addition to the generic message (if set).

For both cases, upgrade can still be forced with -F as usual.

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

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