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 IA-64-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple, override only for 64-bit

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

         CT_TARGET_ARCH="mips64${target_endian_el}"

     else

         # The architecture part of the tuple:

         CT_TARGET_ARCH="${CT_ARCH}${target_endian_el}"

     fi

     # Override CFLAGS for endianness:

     case "${CT_ARCH_BE},${CT_ARCH_LE}" in

         y,) CT_ARCH_ENDIAN_CFLAG="-EB";;

         ,y) CT_ARCH_ENDIAN_CFLAG="-EL";;

     esac

     # Override ABI flags

     CT_ARCH_ABI_CFLAG="-mabi=${CT_ARCH_mips_ABI}"

     CT_ARCH_WITH_ABI="--with-abi=${CT_ARCH_mips_ABI}"

 }