treewide: provide global default for SUPPORTED_DEVICES

The majority of our targets provide a default value for the variable
SUPPORTED_DEVICES, which is used in images to check against the
compatible on a running device:

  SUPPORTED_DEVICES := $(subst _,$(comma),$(1))

At the moment, this is implemented in the Device/Default block of
the individual targets or even subtargets. However, since we
standardized device names and compatible in the recent past, almost
all targets are following the same scheme now:

  device/image name:  vendor_model
  compatible:         vendor,model

The equal redundant definitions are a symptom of this process.

Consequently, this patch moves the definition to image.mk making it
a global default. For the few targets not using the scheme above,
SUPPORTED_DEVICES will be defined to a different value in
Device/Default anyway, overwriting the default. In other words:
This change is supposed to be cosmetic.

This can be used as a global measure to get the current compatible
with: $(firstword $(SUPPORTED_DEVICES))
(Though this is not precisely an achievement of this commit.)

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

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