log: Handle line continuation

When multiple log() calls are used which don't end in newline, the
log prefix is prepended multiple times in the same line. This makes the
output look strange.

Fix this by detecting when the previous log record did not end in newline.
In that case, setting a flag.

Drop the unused BUFFSIZE in the test while we are here.

As an example implementation, update log_console to check the flag and
produce the expected output.

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