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>

 # Build script for automake

 CT_AUTOMAKE_VERSION=1.11.1

 do_companion_tools_automake_get() {

     CT_GetFile "automake-${CT_AUTOMAKE_VERSION}" \

                {ftp,http}://ftp.gnu.org/gnu/automake

 }

 do_companion_tools_automake_extract() {

     CT_Extract "automake-${CT_AUTOMAKE_VERSION}"

     CT_Patch "automake" "${CT_AUTOMAKE_VERSION}"

 }

 do_companion_tools_automake_build() {

     CT_DoStep EXTRA "Installing automake"

     mkdir -p "${CT_BUILD_DIR}/build-automake"

     CT_Pushd "${CT_BUILD_DIR}/build-automake"

     CT_DoExecLog CFG \

     "${CT_SRC_DIR}/automake-${CT_AUTOMAKE_VERSION}/configure" \

         --prefix="${CT_TOOLS_OVERIDE_DIR}"

     CT_DoExecLog ALL make

     CT_DoExecLog ALL make install

     CT_Popd

     CT_EndStep

 }