stm32f1: Use internal functions for early USB gpio toggle

Avoid using the "low-level" library timing utilities.  This is in
preparation for using SysTick as part of the timer implementation.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/stm32f1/internal.h

@@ -8,4 +8,6 @@
 extern GPIO_TypeDef *const digital_regs[];
 extern uint32_t const digital_pins[];
 
+void udelay(uint32_t usecs);
+
 #endif // internal.h