dist, dev: fix etherswitch upgrade script

dist, dev: fix etherswitch upgrade script

The aforementioned upgrader in [1] assumes every option in [system]
has a delimiting '.', and also seems to do its rewriting work a bit too
unconditionally.  Most checkpoints in the wild don't have this device,
in which case this script should be a safe no-op.

[1] 2aa4d7b  dist, dev: Fixed the packet ordering in etherswitch

Change-Id: Icfd0350985109df1628eb9ab864cda42c54060a8
Reviewed-by: Gabor Dozsa <gabor.dozsa@arm.com>