SoftwareSerialM for SKR STM32F1 boards (#15875)

Marlin/src/HAL/HAL_STM32F1/HAL.cpp

@@ -272,7 +272,7 @@ extern "C" {
 // return free memory between end of heap (or end bss) and whatever is current
 
 /*
-#include "wirish/syscalls.c"
+#include <wirish/syscalls.c>
 //extern caddr_t _sbrk(int incr);
 #ifndef CONFIG_HEAP_END
 extern char _lm_heap_end;

Marlin/src/HAL/HAL_STM32F1/SoftwareSerial.h

@@ -20,7 +20,9 @@
 
 #include <stdint.h>
 
-#define SW_SERIAL_PLACEHOLDER 1
+#ifndef HAVE_SW_SERIAL
+   #define SW_SERIAL_PLACEHOLDER 1
+#endif
 
 class SoftwareSerial {
 public:

Marlin/src/HAL/HAL_STM32F1/inc/SanityCheck.h

@@ -36,3 +36,8 @@
 #if ENABLED(FAST_PWM_FAN)
   #error "FAST_PWM_FAN is not yet implemented for this platform."
 #endif
+
+#if !defined(HAVE_SW_SERIAL) && HAS_TMC220x
+  #warning "With TMC2208/9 consider using SoftwareSerialM with HAVE_SW_SERIAL and appropriate SS_TIMER."
+  #error "Missing SoftwareSerial implementation."
+#endif