blackboxPrintf - return number of characters written

change fmt to `const char*`

src/main/blackbox/blackbox_io.c

@@ -91,12 +91,13 @@ static void _putc(void *p, char c)
 }
 
 //printf() to the blackbox serial port with no blocking shenanigans (so it's caller's responsibility to not write too fast!)
-void blackboxPrintf(char *fmt, ...)
+int blackboxPrintf(const char *fmt, ...)
 {
     va_list va;
     va_start(va, fmt);
-    tfp_format(NULL, _putc, fmt, va);
+    int written = tfp_format(NULL, _putc, fmt, va);
     va_end(va);
+    return written;
 }
 
 // Print the null-terminated string 's' to the serial port and return the number of bytes written

src/main/blackbox/blackbox_io.h

@@ -36,7 +36,7 @@ uint8_t blackboxWriteChunkSize;
 
 void blackboxWrite(uint8_t value);
 
-void blackboxPrintf(char *fmt, ...);
+int blackboxPrintf(const char *fmt, ...);
 int blackboxPrint(const char *s);
 
 void blackboxWriteUnsignedVB(uint32_t value);