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>

 # Menu for the cross GDB

 config STATIC_TOOLCHAIN

     select GDB_CROSS_STATIC if GDB_CROSS

 config GDB_CROSS

     bool

     prompt "Cross-gdb"

     default y

     select GDB_GDBSERVER if ! BARE_METAL

     help

       Build and install a cross-gdb for the target, to run on host.

 if GDB_CROSS

 config GDB_CROSS_STATIC

     bool

     prompt "Build a static cross gdb"

     default n

     help

       A static cross gdb can be usefull if you debug on a machine that is

       not the one that is used to compile the toolchain.

       That way, you can share the cross-gdb without installing a toolchain

       on every machine that will be used to debug target programs.

 endif # GDB_CROSS