[sl-core] Enhance the handling of MT profiles.

* tools/bin/slr.in: Ensure that `SIMPROFILE' is defined before
loading the profile, because it might use the variable.
[sl-core] Fix an MT initialization bug.

The bug was introduced in January 2013
(ef5194dcd8158392d18590b254e1df0d91f327c3) when dropping support
for slr variables.

* tools/lib/host/load.c,
* tools/lib/host/slrt.c: Declare `slr_init' with two arguments,
not three.
[sl-core] Report time to completion when main() returns.

* lib/mgos/init/mtinit.c (sys_init, sys_cleanup): Measure and
report the execution time.
* tools/lib/host/load.c,
* tools/lib/mt/mta/slrt.s,
* tools/lib/mt/mts/slrt.s,
* tools/lib/mt/mipsel/slrt.s: Ensure `sys_cleanup' is called.