Add error if OS tickrate is changed

The current mbed-os drivers rely on a tickrate of 1ms for timing.
This means that if OS_TICK_FREQ is set to any value other than 1000
then mbed-os driver will no longer delay for the correct amount of
time. To prevent this from happening this patch triggers a compile
time error if a tickrate other than 1m is used.

Author: c1728p9

rtos/TARGET_CORTEX/mbed_rtx_conf.h

@@ -41,6 +41,10 @@
 
 #define OS_DYNAMIC_MEM_SIZE         0
 
+#if defined(OS_TICK_FREQ) && (OS_TICK_FREQ != 1000)
+#error "OS Tickrate must be 1000 for system timing"
+#endif
+
 #if defined (__CC_ARM) || (defined (__ARMCC_VERSION) && (__ARMCC_VERSION >= 6010050))
 /* ARM toolchain uses up to 8 static mutexes, any further mutexes will be allocated on the heap. */
 #define OS_MUTEX_OBJ_MEM            1