eeprom: provide smaller code for SERIAL_ECHOPAIR

SERIAL_ECHOPAIR implies, eventually, two calls to MYSERIAL.print.  One
of these has FORCE_INLINE for a per-character loop, and both involve
constructing a method call rather than a simple function call.

Produce better and smaller code by providing three specialised
functions serial_echopair.  This saves 672 bytes of program memory
(with EEPROM_SETTINGS and SDSUPPORT enabled).

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

Marlin/Marlin.h

 #define SERIAL_ECHOLN(x) SERIAL_PROTOCOLLN(x)
 #define SERIAL_ECHOLNPGM(x) SERIAL_PROTOCOLLNPGM(x)
 
-#define SERIAL_ECHOPAIR(name,value) {SERIAL_ECHOPGM(name);SERIAL_ECHO(value);}
+#define SERIAL_ECHOPAIR(name,value) (serial_echopair_P(PSTR(name),(value)))
+
+void serial_echopair_P(const char *s_P, float v);
+void serial_echopair_P(const char *s_P, double v);
+void serial_echopair_P(const char *s_P, unsigned long v);
 
 
 //things to write to serial from Programmemory. saves 400 to 2k of RAM.

Marlin/Marlin.pde

 
 void get_arc_coordinates();
 
+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); }
+
 extern "C"{
   extern unsigned int __bss_end;
   extern unsigned int __heap_start;