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>

 reporter_name="Anthony Foiani <anthony.foiani@gmail.com>"

 reporter_url="http://sourceware.org/ml/crossgcc/2010-09/msg00100.html"

 reporter_comment="

 This is a sample config file for Freescale e500v2 processors (e.g., MPC8548,

 MPC8572). It uses eglibc (for e500/SPE patches) and a recent gcc (4.5.1,

 for e500v2 DPFP support) and will generate appropriate dual-precision

 floating point instructions by default.

 Note: If building a Linux kernel with this toolchain, you will want to make

 sure -mno-spe AND -mspe=no are passed to gcc to prevent SPE ABI/instructions

 from getting into the kernel (which is currently unsupported). At this time,

 the kernel build system properly passes those two options, but older kernels

 were only passing -mno-spe by default."