Added User Print - A "light" console out option (#8)

User print disables the normal print messages in the body of QMK/TMK
code and is meant as a lightweight alternative to NOPRINT. Use it when
you only want to do a spot of debugging but lack flash resources for
allowing all of the codebase to print (and store their wasteful
strings).

tmk_core/common/mbed/xprintf.cpp

@@ -7,7 +7,7 @@
 #define STRING_STACK_LIMIT    120
 
 //TODO
-int xprintf(const char* format, ...) { return 0; }
+int __xprintf(const char* format, ...) { return 0; }
 
 #if 0
 /* mbed Serial */