Add a function to output spaces to serial

Marlin/serial.cpp

 void serial_echopair_P(const char* s_P, float v)         { serialprintPGM(s_P); SERIAL_ECHO(v); }
 void serial_echopair_P(const char* s_P, double v)        { serialprintPGM(s_P); SERIAL_ECHO(v); }
 void serial_echopair_P(const char* s_P, unsigned long v) { serialprintPGM(s_P); SERIAL_ECHO(v); }
+
+void serial_spaces(uint8_t count) { while (count--) MYSERIAL.write(' '); }

Marlin/serial.h

 FORCE_INLINE void serial_echopair_P(const char* s_P, bool v) { serial_echopair_P(s_P, (int)v); }
 FORCE_INLINE void serial_echopair_P(const char* s_P, void *v) { serial_echopair_P(s_P, (unsigned long)v); }
 
+void serial_spaces(uint8_t count);
+#define SERIAL_ECHO_SP(C)     serial_spaces(C)
+#define SERIAL_ERROR_SP(C)    serial_spaces(C)
+#define SERIAL_PROTOCOL_SP(C) serial_spaces(C)
+
 //
 // Functions for serial printing from PROGMEM. (Saves loads of SRAM.)
 //