[release,dunecontrol] Use $GREP consistently.
(cherry picked from commit 1d8ba49d48b68d01c4889b3a0bdfd4f68379d916)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>
[dunecontrol,bugfix] Stops using non-posix \s with sed and only uses sed if variable is there.
(cherry picked from commit ca8dc751212677d4071d99a6ca734a50746ebe26)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>