Remove pre-release info

Turns out that get_version and release.sh strategy do not agree
very well on this, which may end up as version comparison problem.

SemVer 2.0 says that pre-release versions go before the associated
versions, that is, 1.0.0-blah goes before 1.0.0, and at the same time
remove the compatibility guarantee.  This means that if pre-releases
are needed, we will probably need to change both PRERELEASE and VERSION,
which is not what was expected when writing release.sh.

For now, the safe thing is to remove PRERELEASE altogether.  If we ever
need to do a pre-release, we will re-verify if it works properly; until
then, build meta-data give us enough luxury.

config.mk

 VERSION = 0.6.2
-PRERELEASE = devel
 CODENAME = StopIteration
 RELSRC = master
 RELDST = last

mkit/include/build.sh

     #         from tags" fails if we are in shallow clone made from
     #         other than a tagged commit.
     #
+    # FIXME:  Using PRERELEASE for release IDs may not be compatible with
+    #         release strategy implemented in release.sh
+    #
     local version=$VERSION
     local prerl=$PRERELEASE
     grep ":" <<<"$prerl" && warn "colon in PRERELEASE may corrupt version data: $prerl"

mkit/include/release.sh

     # Span release routines: make a signed tag, check branch
     # and update release branch
     #
+    # FIXME: Using PRERELEASE as build.sh:get_version() does may not be
+    #        compatible with this release strategy
+    #
     local level=$1
     git rev-parse HEAD >&/dev/null || die "cannot release outside git repo"
     local lastfile=$(git diff-tree --no-commit-id --name-only -r HEAD)