fakedate: new package

`date' is widely used by packages to include build information in their
binaries. Unfortunately, this is incompatible with BR2_REPRODUCIBLE.

Instead of having to identify all `date' invocations in the different
packages, this commit adds a small tool that allows to always return
the same date.

This work was sponsored by `BA Robotic Systems'.

[Peter: drop debugging print]
Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>