toaster.bbclass: read build stats

toaster.bbclass: read build stats

In the process of removing the local system
accesses from toaster UI (which must be able to
run remotely), the code to read build stats
is moved from Bitbake Toaster UI
to the server-side toaster.bbclass

The code will accumulate a list of stat files
to be read at build completion. When the
build completes, the whole data list is read and
sent through in a single event.

    [YOCTO #5604]

(From OE-Core rev: 0c455c0708335eecd1e659680b6cddb4782e80fa)

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