image_types: Fix reproducible builds for initramfs and UKI img

I've encountered issues reproducing initramfs and UKI image builds,
which will be fixed with this patch.

1. initramfs
There's a symbolic link to /sbin/init, which is appended to the cpio archive after creation.
The links timestamp needs to be static and the cpio append command needs the '--reproducible' flag to produce deterministic outcomes.

2. Unified Kernel Image
'--preserve-dates' is required for a static 'Time/Date' entry.
I've added '--enable-deterministic-archives' although in my case this
didn't change anything.

Signed-off-by: Frieder Paape <frieder@konvera.io>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>