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/avr/main.c

@@ -4,9 +4,13 @@
 //
 // This file may be distributed under the terms of the GNU GPLv3 license.
 
+#include "autoconf.h" // CONFIG_MCU
+#include "command.h" // DECL_CONSTANT
 #include "irq.h" // irq_enable
 #include "sched.h" // sched_main
 
+DECL_CONSTANT(MCU, CONFIG_MCU);
+
 // Main entry point for avr code.
 int
 main(void)