Having dtostrf on AVR available should enable support for %f.

src/dtostrf.c

@@ -0,0 +1,40 @@
+/*
+  dtostrf - Emulation for dtostrf function from avr-libc
+  Copyright (c) 2016 Arduino LLC.  All rights reserved.
+
+  This library is free software; you can redistribute it and/or
+  modify it under the terms of the GNU Lesser General Public
+  License as published by the Free Software Foundation; either
+  version 2.1 of the License, or (at your option) any later version.
+
+  This library is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public
+  License along with this library; if not, write to the Free Software
+  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+
+// This is a default implementation for dtostrf function.
+// This file should be used if the standard lib doesn't provide an
+// implementation of dtostrf.
+
+// Create a file called "dtostrf.c" with the following include:
+// #include "api/deprecated-avr-comp/avr/dtostrf.c.impl"
+
+#if defined(ARDUINO_ARCH_AVR)
+
+#include <stdio.h>
+
+char *dtostrf (double val, signed char width, unsigned char prec, char *sout) {
+  asm(".global _printf_float");
+
+  char fmt[20];
+  sprintf(fmt, "%%%d.%df", width, prec);
+  sprintf(sout, fmt, val);
+  return sout;
+}
+
+#endif /* ARDUINO_ARCH_AVR */