scripts/lib/buildstats: handle top-level build_stats not being complete

scripts/lib/buildstats: handle top-level build_stats not being complete

If we try to parse a buildstats directory which was either aborted or
is still being built then the top-level build_stats file doesn't
contain an elapsed value which causes an exception:

UnboundLocalError: local variable 'elapsed' referenced before assignment

Default both start and elapsed to 0 so that the parse succeeds.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 701d985aa8f2e9c2b9c0736fa25b424f3701889e)
Signed-off-by: Steve Sakoman <steve@sakoman.com>