gluon-core, gluon-site: get version numbers from .config

Calling git describe directly breaks isolation between the build system
and packages. Replace this with proper .config variables, like we
already do for GLUON_RELEASE.

Also replace the PKG_VERSION hack with a static '1', as we do for other
packages - while having those version numbers in opkg was cute, it was
also entirely useless. Having a fixed PKG_VERSION allows us to remove
the PKG_BUILD_DIR override as well.