build: add DEVICE_COMPAT_VERSION and DEVICE_COMPAT_MESSAGE

build: add DEVICE_COMPAT_VERSION and DEVICE_COMPAT_MESSAGE

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. It will be implemented via a per-device
Make variable DEVICE_COMPAT_VERSION, which will be set to 1.0
globally by default and then can be overwritten as needed.

Furthermore, a variable DEVICE_COMPAT_MESSAGE is added, where
a message to be displayed during sysupgrade may be specified
optionally.

This patch only implements the build variables and adds them
to the sysupgrade metadata, the evaluation will be addressed
in a subsequent patch.

To set it, one would just need to add the following to a device node:

define Device/somedevice
  ...
  DEVICE_COMPAT_VERSION := 1.1
  DEVICE_COMPAT_MESSAGE := Config cannot be migrated from swconfig to DSA
endef

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