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

 # PPL options

 choice

     bool

     prompt "PPL version"

 # Don't remove next line

 # CT_INSERT_VERSION_BELOW

 config PPL_V_0_10_2

     bool

     prompt "0.10.2"

 endchoice

 config PPL_VERSION

     string

 # Don't remove next line

 # CT_INSERT_VERSION_STRING_BELOW

     default "0.10.2" if PPL_V_0_10_2