build: improve support for custom builds

Add a counter field to RELEASE, just before the date, and fix it at zero.
This allows custom package builds to override it in a way that sorts before
the official packages.

Example:

  Official release:   1.6.0-0.20160120.<githash>
  Custom release 1:   1.6.0-1.avi.20160121.<githash>
  Custom release 2:   1.6.0-2.avi.20160122.<githash>

The counter (0/1/2) ensures that the build number dominates over the date
when sorting.

Message-Id: <20170122102814.19649-1-avi@scylladb.com>

SCYLLA-VERSION-GEN

@@ -10,7 +10,12 @@ else
 	DATE=$(date +%Y%m%d)
 	GIT_COMMIT=$(git log --pretty=format:'%h' -n 1)
 	SCYLLA_VERSION=$VERSION
-	SCYLLA_RELEASE=$DATE.$GIT_COMMIT
+	# For custom package builds, replace "0" with "counter.your_name",
+	# where counter starts at 1 and increments for successive versions.
+	# This ensures that the package manager will select your custom
+	# package over the standard release.
+	SCYLLA_BUILD=0
+	SCYLLA_RELEASE=$SCYLLA_BUILD.$DATE.$GIT_COMMIT
 fi
 
 echo "$SCYLLA_VERSION-$SCYLLA_RELEASE"