build: explicitly unexport CONFIG_SITE

On systems with CONFIG_SITE in the environment (e.g. OpenSUSE) make will export
the CONFIG_SITE set in include/package.mk by default. This will cause host
builds to get the target site configuration, leading to all kinds of weirdness
(wrong pointer size, wrong endianess).

Fix this by explicitly unexporting CONFIG_SITE. The explicit export for the
target builds overrides the unexport, so the target builds will still correctly
get the site config.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

SVN-Revision: 44019