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 -durN gcc-4.2.1.orig/gcc/config.gcc gcc-4.2.1/gcc/config.gcc

--- gcc-4.2.1.orig/gcc/config.gcc	2007-02-03 06:25:20.000000000 +0100

+++ gcc-4.2.1/gcc/config.gcc	2007-08-03 20:29:52.000000000 +0200

@@ -1964,7 +1964,7 @@

         ;;

 sh-*-elf* | sh[12346l]*-*-elf* | sh*-*-kaos* | \

 sh-*-symbianelf* | sh[12346l]*-*-symbianelf* | \

-  sh-*-linux* | sh[346lbe]*-*-linux* | \

+  sh*-*-linux* | sh[346lbe]*-*-linux* | \

   sh-*-netbsdelf* | shl*-*-netbsdelf* | sh5-*-netbsd* | sh5l*-*-netbsd* | \

    sh64-*-netbsd* | sh64l*-*-netbsd*)

 	tmake_file="${tmake_file} sh/t-sh sh/t-elf"