rtos: Add FPU detection to ChibiOS/RT

The stacking of ChibiOS/RT depends on the usage of an FPU. If the
FPU is enabled the FPU registers are also saved on context switch.

This patch adds automatic detection of FPU for armv7m targets.

Note: With this patch, openocd will only output an error message
      warning that the FPU is enabled.

      For further FPU support, the correct stacking information
      also needs to be added.

Change-Id: I0984cbd9180f247ba2fa610e74a6413cc54239ea
Signed-off-by: Matthias Blaicher <matthias@blaicher.com>
Reviewed-on: http://openocd.zylin.com/961
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>