toaster.bbclass: read elapsed time from the stats file

We read the elapsed time fromt the build stats file, instead
of computing it independently.

[YOCTO #6833]
[YOCTO #6685]

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>