mkimg: Add a reproducible mode

mkimg embeds a UUID in the GPT header and uses the current time of day
to generate it, so its output is not reproducible by default. Add a -R
flag to ask it to use a fixed time for UUID generation.

Merge the FreeBSD-specific implementation of osdep_uuidgen() with that
of Linux, as the use of uuidgen(2) is incompatible with reproducible
output mode.

Reviewed by: bnovkov
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D49716