base-files: do_install.sigdata: remove the depends on DATE

base-files: do_install.sigdata: remove the depends on DATE

If we run "bitbake -S base-files" today, and re-run it tomorrow with
nothing changed, we would see that the do_install.sigdata changes
because of:

do_intall -> do_install_basefilesissue -> DISTRO_VERSION -> DATE

We had set:
IMAGE_NAME[vardepsexclude] += "DATETIME"
in meta/conf/bitbake.conf, we can set a similar line in
base-files_3.0.14.bb to fix the problem.

[YOCTO #6032]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>