tunneldigger: set PKG_SOURCE_DATE

tunneldigger: set PKG_SOURCE_DATE

opkg requires monotonically increasing version numbers to know which
version of a package is newer. As git commit IDs do not satisfy this
condition, PKG_SOURCE_DATE must be set to the date of the referenced
commit, resulting in the complete version number '2021-03-08-4f72b305-1'.

As the source date also becomes part of the paths inside the download
archive, the source hash must be updated as well.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
(cherry picked from commit 0a3e5dd122abb92f215369eeb0a957114b61746f)