rtos: removed chSysInit from detection of ChibiOS (#121)

rtos: removed chSysInit from detection of ChibiOS (#121)

Using the presence of the 'chSysInit' symbol for detection of
ChibiOS is dangerous because this symbol may not be available
if link-time optimisation is used.

This patch removes this reliance, so the symbols 'ch' and 'ch_debug'
are the only things required for ChibiOS detection.

If 'ch' is present but 'ch_debug' is not, an info message suggests
that Chibios might be present without its registry being enabled.
This message has been reworded a little to make it slightly more
equivocal because the chances of a false positive message are
increased.

Addresses bug #121, "ChibiOS rtos detection fails with LTO enabled".

Change-Id: I5ef224735c06446751adee010ce75be4f30f0403
Signed-off-by: Andy Pomfret <cooperised@gmail.com>
Reviewed-on: http://openocd.zylin.com/3381
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>