usb: FREE_STANDING should come with USE_SYS.

mcu/ABOUT-USB

@@ -7,3 +7,7 @@ powered system, we need to define the hardware interface detecting
 VBUS voltage.  Only after detecting VBUS power, we can enable USB
 driver (D+/D- lines).  For self powered system, driving D+/D- lines
 (by fixed pull-up resistor) violates the USB specification.
+
+This driver can be used without Chopstx.  Please supply
+-DFREE_STANDING when used without Chopstx.  An example can be find
+in gnuk/regnual.

mcu/usb-stm32f103.c

@@ -31,6 +31,10 @@
 
 #ifdef USE_SYS
 #include "sys-stm32f103.h"
+#else
+# ifdef FREE_STANDING
+# error "Please compile with -DUSE_SYS"
+# endif
 #endif
 
 #include "stm32f103.h"