kernel-yocto: allow patch author date to be commit date

kernel-yocto: allow patch author date to be commit date

In situations where a buid needs to be reproducible, it is
sometimes desireable to use a patches author date, versus the
time when it is applied. This generates a consistent hash
between different patch applications.

We leverage the existing KERNEL_DEBUG_TIMESTAMPS to trigger
the use of a new option to kgit-s2q. This allows us to use
the author date in a reproducible configuration, but disable
it if we need the current time/date.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>