Implement tickless sleeping

Energy-wise it would be wise to sleep ticklessly.

changed due date to July 04, 2019

Implemented in 0ca995ad. There might still be some issues related to preemption and interrupt handling as described in 258d5963.

Ref https://git.card10.badge.events.ccc.de/card10/firmware/blob/master/epicardium/support.c#L18-35:

/*
 * WFE because the other core should be able to notify
 * epicardium if it wants to issue an API call.
 */

/*
 * TODO: Ensure this is actually correct and does not have any
 * race conditions.
 */
__asm volatile( "cpsie i" ::: "memory" );
__asm volatile( "dsb" ::: "memory" );
__asm volatile( "isb" );
__asm volatile( "wfe" );
__asm volatile( "dsb" ::: "memory" );
__asm volatile( "isb" );
__asm volatile( "cpsid i" ::: "memory" );
__asm volatile( "dsb" );
__asm volatile( "isb" );

closed via commit 5054fdeb

mentioned in issue #71 (closed)