buildtools: store path to buildtools in a variable

Currently, the buildtools are installed relative to ${CT_PREFIX_DIR}.
Change that by introducing ${CT_BUILDTOOLS_DIR}, which is
still set relative to ${CT_PREFIX_DIR}, but which will make it easy
to change in the future.

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

This is NOT the official version of dialog.  This version has been

significantly modified from the original.  It is for use by the Linux

kernel configuration script.  Please do not bother Savio Lam with 

questions about this program.