STM32F1: AUTO_BED_LEVELING_UBL build fixes (#15240)

Marlin/src/HAL/shared/Marduino.h

     //#define strchr_P(s,c) strchr(s,c)
   #endif
 
+  #ifndef snprintf_P
+    #define snprintf_P snprintf
+  #endif
   #ifndef vsnprintf_P
     #define vsnprintf_P vsnprintf
   #endif

Marlin/src/core/serial.h

 #define SERIAL_PRINTF(V...)     SERIAL_OUT(printf, V)
 #define SERIAL_FLUSH()          SERIAL_OUT(flush)
 
-#if TX_BUFFER_SIZE > 0
+#ifdef __STM32F1__
+  #define SERIAL_FLUSHTX()      SERIAL_OUT(flush)
+#elif TX_BUFFER_SIZE > 0
   #define SERIAL_FLUSHTX()      SERIAL_OUT(flushTX)
 #else
   #define SERIAL_FLUSHTX()