netcf: fix offline rebuilding failed

The netcf downloaded gnulib source to ${S} at the configure time,
while the network disconnected, if sstate cache was cleaned, the
netcf rebuilding failed.

Added a new recipe named 'gnulib' to download gnulib source to
${GITDIR} and populate it to the staging data dir.

So netcf could specify this local directory where gnulib sources
reside to save the bandwidth downloading.

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>