scripts: unset CONFIG_SITE

Some distributions (eg. openSUSE 12.1) systematically export
the CONFIG_SITE environment variable to point to a custom
script setting misc paths for ./configure.

This can, and does, break when cross-compiling for architectures
that are not supported by this script.

The simple workaround is to unset this variable.
NB: buildroot has a similar fix:
http://git.buildroot.org/buildroot/commit/?id=12c9f7dd6dee9c6029b4f9a12d6aac1516911ab4

Reported-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 21f4f28e60ec0342133086c9a51e7f0e5b181fb8)

diff -dur binutils-2.17.old/bfd/opncls.c binutils-2.17/bfd/opncls.c

--- binutils-2.17.old/bfd/opncls.c	2006-03-16 13:20:16.000000000 +0100

+++ binutils-2.17/bfd/opncls.c	2007-05-01 18:26:11.000000000 +0200

@@ -158,6 +158,13 @@

 {

   bfd *nbfd;

   const bfd_target *target_vec;

+  struct stat s;

+

+  if (stat (filename, &s) == 0)

+    if (S_ISDIR(s.st_mode)) {

+      bfd_set_error (bfd_error_file_not_recognized);

+      return NULL;

+    }

   nbfd = _bfd_new_bfd ();

   if (nbfd == NULL)