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 powerpc-specific values

 CT_DoArchTupleValues () {

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

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

         CT_TARGET_ARCH="powerpc64"

     fi

     # Add spe in the tuple if needed

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

         glibc,|eglibc,)   CT_TARGET_SYS=gnu;;

         glibc,y|eglibc,y) CT_TARGET_SYS=gnuspe;;

     esac

     # Add extra flags for SPE if needed

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

         CT_ARCH_TARGET_CFLAGS="-mabi=spe -mspe"

         CT_ARCH_CC_CORE_EXTRA_CONFIG="--enable-e500_double"

         CT_ARCH_CC_EXTRA_CONFIG="--enable-e500_double"

     fi

 }