synergy: fix resolving of Buildroot git revision

synergy: fix resolving of Buildroot git revision

Starting version v1.10.0-stable, Synergy tries to resolve the git revision in
the build directory. This causes CMake to either detect the Buildroot commit
or fail when no git repo is found.

By manually setting SYNERGY_VERSION_STAGE to another value then snapshot, this
behavior is skipped.

Fixes:
  http://autobuild.buildroot.net/results/5bab942322357bd71901cfc13ed3cff0c17edcce

Signed-off-by: Pieterjan Camerlynck <pieterjan.camerlynck@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>