release: Set default timestamps for makefs and mkimg outputs

[markj]

Summary

This is a step towards making release build artifacts reproducible,
assuming that the src tree was built with WITH_REPRODUCIBLE_BUILD set.

In particular, this change makes the use of reproducible timestamps the
default behaviour. We could alternately make this conditional on an
environment variable, but it seems to me that we should simply configure
this by default.

[jrtc27]

Should this not be tied to SOURCE_DATE_EPOCH rather than some arbitrary and misleading timestamp (1st Jan 2024, which predates any release that will include this code)?

[markj]

Use GITEPOCH, i.e., the commit time of the most recent commit to provide the timestamp.