scripts: getver.sh: approximate version from date

scripts: getver.sh: approximate version from date

When doing package support and management it is often the case that
knowing the corresponding openwrt repo's release version is useful.

For example, when adding package changes to the ASU server, the
openwrt revision is used as the cutoff for applying those changes.
Knowing a package change's hash in its remote feed repo allows us
to look up its change date, which we can now use with getver.sh
to approximate the revision in openwrt at which it was made.

Signed-off-by: Eric Fahlgren <ericfahlgren@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/17817
Signed-off-by: Robert Marko <robimarko@gmail.com>