In mkit's world, last change before release (ie. the commit that gets
tagged) must be nothing but version bump in "config.mk"

Other checks that rely on version number that is derived from this tag
would then give weird messages.

Folow-up to 37b5db9 and 9c94a46.

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.