blackpill-f4: Provision platform-specific bits for ENABLE_DEBUG=1

* Flip `SCS->DEMCR |= MON_EN` for DebugMon to catch semihosting breakpoints
  when there is no upstream debugger and we want to log
* Declare PLATFORM_HAS_DEBUG to pull conditionally compiled statements

src/platforms/common/blackpill-f4/blackpill-f4.c

@@ -29,6 +29,7 @@
 
 #include <libopencm3/stm32/rcc.h>
 #include <libopencm3/cm3/scb.h>
+#include <libopencm3/cm3/scs.h>
 #include <libopencm3/cm3/nvic.h>
 #include <libopencm3/stm32/exti.h>
 #include <libopencm3/stm32/usart.h>
@@ -103,6 +104,10 @@ void platform_init(void)
 #endif
 
  platform_timing_init();
+#if ENABLE_DEBUG == 1
+ /* Allow vectoring to DebugMon exception handler upon semihosting breakpoints */
+ SCS_DEMCR |= SCS_DEMCR_VC_MON_EN;
+#endif
  blackmagic_usb_init();
  aux_serial_init();
 

src/platforms/common/blackpill-f4/blackpill-f4.h

@@ -29,6 +29,11 @@
 
 #define PLATFORM_HAS_TRACESWO
 
+#if ENABLE_DEBUG == 1
+#define PLATFORM_HAS_DEBUG
+extern bool debug_bmp;
+#endif
+
 /*
  * If the SHIELD macro is passed to make, other macros are defined.
  * Build the code using `make PROBE_HOST=blackpill-f4x1cx SHIELD=1` to define the SHIELD macro.