siteinfo: Use configuration caches when available

Generate cached configuration data for autoconf for the package
based on a list of header, types, and functions to eliminate the
need for all subesequent package builds to do the actual tests
via the cross compiler and sysroot.  The cache files are
stored in the sysroot in ${STAGING_DATADIR}/${TARGET_SYS}_config_site.d.
Siteinfo appends any files it finds in that directory to the normal
CONFIG_SITE.  All of the cache values set the variables only if not
already set so they may be overridden by any of the normal site files.

Signed-off-by: Jeff Polk <jeff.polk@windriver.com>