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 ARM-specific values

CT_DoArchTupleValues() {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}${target_endian_eb}"

    # The system part of the tuple:

    case "${CT_LIBC},${CT_ARCH_ARM_EABI}" in

        *glibc,y)   CT_TARGET_SYS=gnueabi;;

        uClibc,y)   CT_TARGET_SYS=uclibcgnueabi;;

        *,y)        CT_TARGET_SYS=eabi;;

    esac

    # Set the default instrcution set mode

    case "${CT_ARCH_ARM_MODE}" in

        arm)    ;;

        thumb)

            CT_ARCH_CC_CORE_EXTRA_CONFIG="--with-mode=thumb"

            CT_ARCH_CC_EXTRA_CONFIG="--with-mode=thumb"

#            CT_ARCH_TARGET_CFLAGS="-mthumb"

            ;;

    esac

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

        CT_ARCH_TARGET_CFLAGS+=" -mthumb-interwork"

    fi

}