include: clean package staging dir files before configure

This is required to allow packages to ship libraries with a different API.
Previously the linker would favor the old shared libraries in STAGING_DIR
instead of the just compiled ones in PKG_BUILD_DIR.

Avoid that problem by clearing the package staging files right after
prepare.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 45759