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>

 Original patch from Gentoo:

 gentoo/src/patchsets/binutils/2.19/76_all_use-new-ld-dtags.patch

 diff -durN binutils-2.19.orig/ld/ldmain.c binutils-2.19/ld/ldmain.c

 --- binutils-2.19.orig/ld/ldmain.c	2008-11-23 16:54:42.000000000 +0100

 +++ binutils-2.19/ld/ldmain.c	2008-11-23 16:56:16.000000000 +0100

 @@ -262,6 +262,7 @@

    link_info.allow_undefined_version = TRUE;

    link_info.keep_memory = TRUE;

 +  link_info.new_dtags = TRUE;

    link_info.combreloc = TRUE;

    link_info.strip_discarded = TRUE;

    link_info.emit_hash = TRUE;