Enable putchar for all arm platforms

Instead of having to #undef putchar in every file where we want to use it, we undef it across the board. See the comment in arm-def.h as to why this needs done.

arch/cpu/arm/arm-def.h

@@ -60,6 +60,25 @@ typedef uint32_t uip_stats_t;
 typedef uint32_t rtimer_clock_t;
 #define RTIMER_CLOCK_DIFF(a, b)     ((int32_t)((a) - (b)))
 /** @} */
+
+/*
+ * The stdio.h that ships with the arm-gcc toolchain does this:
+ *
+ * int  _EXFUN(putchar, (int));
+ * [...]
+ * #define  putchar(x)  putc(x, stdout)
+ *
+ * This causes us a lot of trouble: For platforms using this toolchain, every
+ * time we use putchar we need to first #undef putchar. What we do here is to
+ * #undef putchar across the board. The resulting code will cause the linker
+ * to search for a symbol named putchar and this allows us to use the
+ * implementation under os/lib/dbg-io.
+ *
+ * This will fail if stdio.h is included before contiki.h, but it is common
+ * practice to include contiki.h first
+ */
+#include <stdio.h>
+#undef putchar
 /*---------------------------------------------------------------------------*/
 #endif /* ARM_DEF_ */
 /*---------------------------------------------------------------------------*/

arch/cpu/cc2538/dbg.c

@@ -48,9 +48,6 @@
 #define flush()
 #endif
 /*---------------------------------------------------------------------------*/
-#undef putchar
-#undef puts
-
 #define SLIP_END     0300
 /*---------------------------------------------------------------------------*/
 int

os/lib/dbg-io/putchar.c

@@ -35,7 +35,6 @@
 #include <stdio.h>
 #include <string.h>
 /*---------------------------------------------------------------------------*/
-#undef putchar
 #undef putc
 /*---------------------------------------------------------------------------*/
 int