Fix systick when yield overridden

teensy3/EventResponder.cpp

@@ -320,6 +320,19 @@ void MillisTimer::runFromTimer()
 	}
 }
 
+// Long ago you could install your own systick interrupt handler by just
+// creating your own systick_isr() function.  No longer.  But if you
+// *really* want to commandeer systick, you can still do so by writing
+// your function into the RAM-based vector table.
+//
+//   _VectorsRam[15] = my_systick_function;
+//
+// However, for long-term portability, use a MillisTimer object to
+// generate an event every millisecond, and attach your function to
+// its EventResponder.  You can attach as a software interrupt, so your
+// code will run at lower interrupt priority for better compatibility
+// with libraries using mid-to-high priority interrupts.
+
 extern "C" volatile uint32_t systick_millis_count;
 void systick_isr(void)
 {

teensy3/mk20dx128.c

@@ -145,7 +145,7 @@ void usage_fault_isr(void)	__attribute__ ((weak, alias("fault_isr")));
 void svcall_isr(void)		__attribute__ ((weak, alias("unused_isr")));
 void debugmonitor_isr(void)	__attribute__ ((weak, alias("unused_isr")));
 void pendablesrvreq_isr(void)	__attribute__ ((weak, alias("unused_isr")));
-void systick_isr(void)		__attribute__ ((weak, alias("unused_isr")));
+void systick_isr(void);
 
 void dma_ch0_isr(void)		__attribute__ ((weak, alias("unused_isr")));
 void dma_ch1_isr(void)		__attribute__ ((weak, alias("unused_isr")));