Fix Print::printf implementation

[tsl0922]

Requirement: h2zero/platform-n-able#3.

Fixes #62.

[h2zero]

Thanks! Only concern I have is that it will likely cause a stack overflow for constrained devices, nRF51 and nRF52810 etc.. I will test it and see.

[tsl0922]

https://github.com/esp8266/Arduino/blob/bb79e9076e8402c81c24efbabd19f1baab1c6bd1/cores/esp8266/Print.cpp#L50-L71

The code above maybe better, it uses a smaller buffer first, alloc if the buffer overflow.

[tsl0922]

Follow up to #68 (comment)

I have a working implementation now, if you are OK with the changes, I can open another PR to replace this one @h2zero

--- a/builder/frameworks/arduino/nrf5.py
+++ b/builder/frameworks/arduino/nrf5.py
@@ -71,6 +71,8 @@ env.Append(

     CPPPATH=[
         join(FRAMEWORK_DIR, "cores", board.get("build.core")),
+        join(FRAMEWORK_DIR, "cores", board.get("build.core"),
+            "libc", "printf"),
         join(FRAMEWORK_DIR, "cores", board.get("build.core"),
              "nordic", "nrfx"),
         join(FRAMEWORK_DIR, "cores", board.get("build.core"),

--- a/cores/nRF5/Print.cpp
+++ b/cores/nRF5/Print.cpp
@@ -22,6 +22,7 @@
 #include <string.h>
 #include <math.h>
 #include "Arduino.h"
+#include "printf.h"

 #include "Print.h"

@@ -187,11 +188,16 @@ size_t Print::println(const Printable& x)
   return n;
 }

+extern "C" void streamOutput(char character, void* arg)
+{
+  (static_cast<Print *>(arg))->write(character);
+}
+
 int Print::printf(const char* format, ...)
 {
   va_list va;
   va_start(va, format);
-  int ret = vprintf(format, va);
+  int ret = vfctprintf(&streamOutput, this, format, va);
   va_end(va);
   return ret;
 }

[h2zero]

@tsl0922 That is probably the better solution, please open PR's for this.

[tsl0922]

@tsl0922 That is probably the better solution, please open PR's for this.

Done. This PR has been updated, PR for platform: h2zero/platform-n-able#3.

[h2zero]

Looks good, just need to fix a couple things.

[h2zero]

LGTM, Thanks!