log: Set up a flag byte for log records

log: Set up a flag byte for log records

At present only a single flag (force_debug) is used in log records. Before
adding more, convert this into a bitfield, so more can be added without
using more space.

To avoid expanding the log_record struct itself (which some drivers may
wish to store in memory) reduce the line-number field to 16 bits. This
provides for up to 64K lines which should be enough for anyone.

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