ci/bare-metal: Truncate printed times and prefixes

The date isn't interesting, since you can tell if it wraps around
anyway. Just print the time down to the millisecond, which should be
plenty enough. We also don't need to print 'R SERIAL_CPU> ' when almost
every line is reading from the CPU serial buffer, so make that the
default.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31602>

.gitlab-ci/bare-metal/poe_run.py

@@ -35,7 +35,7 @@ class PoERun:
         self.powerup = args.powerup
         self.powerdown = args.powerdown
         self.ser = SerialBuffer(
-            args.dev, "results/serial-output.txt", "")
+            args.dev, "results/serial-output.txt", ": ")
         self.boot_timeout = boot_timeout
         self.test_timeout = test_timeout
         self.logger = logger