buildtools: the buildtools dir is in fact a prefix

Consider the buildtools install directory as a prefix directory,
that is, install buildtools in prefix/bin/, not in prefix/.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Compute M68k-specific values

 CT_DoArchTupleValues() {

     :

 }