AVR: Explicitly disable interrupts as the first step of system init

Disabling interrupts at the very beginning of system init does not
make sense after reset since interrupts are disabled anyway. But
it is very convenient for debugging purposes, in situations when the
system misbehaves and ocassionally jumps to zero.

arch/avr/src/atmega/atmega_head.S

@@ -368,6 +368,9 @@ vectortab:
 	.func	__start
 __start:
 
+	/* no interrupts! Useful for software reset by jumping to address 0 */
+	cli
+
 	/* Clear the zero register, clear the status register and initialize the
 	 * IDLE thread stack
 	 */