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>

 # Bare metal config options

 config KERNEL_bare_metal

     select BARE_METAL

     help

       'Bare metal' refers to those programs that run without any kernel.

       You probably want to say 'y' here if you plan to use your compiler

       to build bootloaders. It is not yet suitable to build Linux kernels,

       though, because the APCI stuff relies on the target C library headers

       being available?!?!...