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)

     1.1 --- a/scripts/crosstool-NG.sh.in	Wed Dec 07 16:15:50 2011 +0800
     1.2 +++ b/scripts/crosstool-NG.sh.in	Mon Dec 12 21:41:16 2011 +0200
     1.3 @@ -41,6 +41,8 @@
     1.4  CT_TestAndAbort "Don't set CXXFLAGS. It screws up the build." -n "${CXXFLAGS}"
     1.5  CT_Test "GREP_OPTIONS screws up the build. Resetting." -n "${GREP_OPTIONS}"
     1.6  export GREP_OPTIONS=
     1.7 +# Workaround against openSUSE 12.1 that breaks ./configure for cross-compilation:
     1.8 +export CONFIG_SITE=
     1.9  
    1.10  # Some sanity checks on paths content
    1.11  for d in            \