netcf: fix gnulib not found in sysroots while using a shared sstate cache

...
./bootstrap: Bootstrapping from checked-out netcf sources...
./bootstrap: consider installing git-merge-changelog from gnulib
./bootstrap: line 641:
tmp/sysroots/qemux86/usr/share/gnulib/gnulib-tool: No such file or
directory
...

The gnulib source was incorrectly populated to ${STAGING_DATADIR}.
While the build shared an existed sstate cache, the gnulib's
${SYSROOT_DESTDIR} was empty, the gnulib source code was not
found in sysroots.

We should use do_install task to install them to ${D}, and the
default do_populate_sysroot task will populate them to
${STAGING_DATADIR}.

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