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>

 # mingw options

 # depends on MINGW32

 config LIBC_mingw

     bool

     select LIBC_SUPPORT_WIN32THREADS

     help

       The de-facto standard for Mingw distributions.

 choice

     bool

     prompt "Mingw runtime version"

 # Don't remove next line

 # CT_INSERT_VERSION_BELOW

 config MINGWRT_V_3_18

     bool

     prompt "3.18"

 config MINGWRT_V_select

     bool

     prompt "Other version"

 endchoice

 config MINGWRT_VERSION

     string

     prompt "Mingw runtime version" if MINGWRT_V_select

 # Don't remove next line

 # CT_INSERT_VERSION_STRING_BELOW

     default "3.18" if MINGWRT_V_3_18

     help

       Enter the version number of the mingw runtime files to use