config: add an option not to remove the destination directory

In certain circumstances, removing the destination/installation directory
is a bad idea. For example, when the build environment is already taking
care of sanitising the build tree, and pre-installs stuff in there, it is
a very bad idea to remove the destination directory.

This happens now in buildroot, as the crostool-NG backend now installs the
toolchain in the common host-tools directory, and pre-install there a few
host-utilities (eg. host-automake and host-gawk).

Provide a config knob to turn on/off the removal of the destination
directory, defaulting to 'y' (previous behavior), and forced to 'n' when
used as a backend.

Reported-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Compute x86-specific values

 # This one really needs a little love! :-(

 CT_DoArchTupleValues() {

     # Override the architecture part of the tuple:

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH=x86_64

     else

         arch="${CT_ARCH_ARCH}"

         [ -z "${arch}" ] && arch="${CT_ARCH_TUNE}"

         case "${arch}" in

             "")                           CT_TARGET_ARCH=i386;;

             i386|i486|i586|i686)          CT_TARGET_ARCH="${arch}";;

             winchip*)                     CT_TARGET_ARCH=i486;;

             pentium|pentium-mmx|c3*)      CT_TARGET_ARCH=i586;;

             pentiumpro|pentium*|athlon*)  CT_TARGET_ARCH=i686;;

             *)                            CT_TARGET_ARCH=i586;;

         esac

     fi

 }