use special faketime timestamp format to freeze time

faketime needs to have a very specific timestamp format in order to freeze
time.  The time needs to be frozen so that the timestamps don't depend on
the speed of the machine that the build process is running on.  See `man
faketime` for more info on the "advanced timestamp format".  Also, force
time to UTC so its always the same on all machines.

https://lists.mayfirst.org/pipermail/guardian-dev/2014-April/003420.html
use faketime 'advanced timestamp format' to make it freeze time

I was using the write timestamp format, but I had forgotten the -f flag.
Adding it make faketime actually freeze time :-D

Thanks to Michael Rogers for spotting that.
force timezone to UTC for busybox

busybox includes a timestamp in its code, set in a C macro called both
BB_BT and AUTOCONF_TIMESTAMP.  BB_BT is set to AUTOCONF_TIMESTAMP in the
build flags.  AUTOCONF_TIMESTAMP is set in busybox/include/autoconf.h. This
timestamp includes the time zone, so make sure it is always UTC.