Add time_elapsed() function, subsuming $local_top_time

Generalize the elapsed time tracking in local-top so that it can be used
elsewhere. This requires some additional quoting in local_device_setup()
comparisons to pass shellcheck.

Now that the reference time is recorded earlier (in init vs. local-top),
the rootdev wait time will now be reduced by however long it it takes to
process init-premount. The belief is that our wait time is sufficiently
long for that to be negligible. Also, this could potentially break any
local-top scripts that use $local_top_time directly. A survey of
the current packages in sid shows no packages that contain a file under
/usr/share/initramfs-tools/scripts/local-top/ that contain "local_top_time".

Signed-off-by: dann frazier <dannf@debian.org>