trace: Update the file header

It seems better to put the TEXT_BASE value in the file header rather than
in an entry record. While it is true that there is a separate base for
pre-relocation, this can be handled by using offsets in the file.

It is useful to have a version number in case we need to change the trace
format again.

Update the header to make these changes.

Signed-off-by: Simon Glass <sjg@chromium.org>