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

}