generic: Move generic parts of sam3x8e timer.c to generic directory

Most of sam3x8e/timer.c is going to be platform agnostic for any board
with standard irq handling.  Move the generic code into a new file
generic/timer.c.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/sam3x8e/main.c

@@ -5,9 +5,12 @@
 // This file may be distributed under the terms of the GNU GPLv3 license.
 
 #include "board/misc.h" // alloc_maxsize
+#include "command.h" // DECL_CONSTANT
 #include "sam3x8e.h" // WDT
 #include "sched.h" // sched_main
 
+DECL_CONSTANT(MCU, "sam3x8e");
+
 
 /****************************************************************
  * watchdog handler