Add Stream::find(char); to SAM core.

This is a feature added to the AVR core here:
arduino@ed1b8eb
It allows using the find method with a single char (arduino#847).

Author: Chris--A

hardware/arduino/sam/cores/arduino/Stream.h

@@ -73,6 +73,8 @@ class Stream : public Print
   bool find(uint8_t *target, size_t length) { return find ((char *)target, length); }
   // returns true if target string is found, false if timed out
 
+  bool find(char target) { return find (&target, 1); }
+  
   bool findUntil(char *target, char *terminator);   // as find but search ends if the terminator string is found
   bool findUntil(uint8_t *target, char *terminator) { return findUntil((char *)target, terminator); }