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>

 # AVR32 specific configuration file

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_USE_MMU

 ## select ARCH_DEFAULT_BE

 ## select ARCH_SUPPORTS_WITH_ARCH

 ## select ARCH_SUPPORTS_WITH_CPU

 ## select ARCH_SUPPORTS_WITH_TUNE

 ## select ARCH_SUPPORTS_WITH_FPU

 ##

 ## help The AVR32 architecture, as defined by:

 ## help     http://www.atmel.com/products/avr32