classes/buildhistory: use BB_CMDLINE to get bitbake command line

classes/buildhistory: use BB_CMDLINE to get bitbake command line

We put the bitbake command line in the buildhistory commit message for
informational purposes, but calling sys.argv to get that as we were
previously doesn't really work in memory resident mode - that gives you
the command used to start the bitbake server which is much less
interesting. Use the just-introduced BB_CMDLINE variable instead.

Part of the fix for [YOCTO #11634].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>