scripts: check for, and warn about an unset CT_PREFIX_DIR

The user shall provide a directory to install the toolchain into.
If he/she does not, this is an error, and shall be detected properly,
rather than relying on failure down the road.

Thanks to "Pedro I. Sanchez" <psanchez@colcan.ca> for pointing out
the issue:
http://sourceware.org/ml/crossgcc/2009-12/msg00011.html

 source config/global.in

 source config/target.in

 source config/toolchain.in

 source config/kernel.in

 source config/binutils.in

 source config/cc.in

 source config/libc.in

 source config/debug.in

 source config/tools.in

 source config/companion_libs.in