bootstage: Warning if space is exhausted

bootstage: Warning if space is exhausted

At present bootstage silently ignores new records if it runs out of
space. It is sometimes obvious by looking at the report, but the IDs are
not contiguous, so it is easy to miss.

Aad a message so that action can be taken.

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