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)

# This file declares functions for bare metal kernel (IE. none)

# Copyright 2008 Yann E. MORIN

# Licensed under the GPL v2. See COPYING in the root of this package

CT_DoKernelTupleValues() {

    # For bare-metal, there is no kernel part in the tuple

    CT_TARGET_KERNEL=

}

do_kernel_get() {

    :

}

do_kernel_extract() {

    :

}

do_kernel_headers() {

    :

}