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>