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>

 reporter_name="Alexander BIGGA"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-07/msg00000.html"

 reporter_comment=""