fix(sys-apps/efunctions): Create functions.sh symlink in postinst.

This makes double sure that the symlink is never removed by INSTALL_MASK
or PKG_INSTALL_MASK. This symlink is so strictly required by random
tools we cannot allow it to ever go missing by mistake.
Merge pull request #274 from marineam/efunctions

fix(sys-apps/efunctions): Create functions.sh symlink in postinst.