SMP: Add spin_trylock().  Use this in conditions where other CPUs need to stopped but we cannot call enter_critical_section.

sam4s-xplained-pro/nsh: Configuration uses old, improper timer timer interface.  CONFIG_TIMER disabled in configuration.>>

SMP: Add logic to avoid a deadlock condition when CPU1 is hung waiting for g_cpu_irqlock and CPU0 is waitin for g_cpu_paused

The examples/qencoder app was trying to init the encoder by a direct call into the board, cheating in a local header to declare the normally unavailable function prototype.

Remove a assertion condition that appears to rarely cause false-alarm assertions.  Teported by Petteri Aimonen

Spinlocks:  Added capability to provide architecture-specific memory barriers.  This was for i.MX6 but does not help with the SMP problems.  It is still a good feature.

This commit adds a new internal interfaces and fixes a problem with three APIs in the SMP configuration.  The new internal interface is sched_cpu_pause(tcb).  This function will pause a CPU if the task associated with 'tcb' is running on that CPU.  This allows a different CPU to modify that OS data stuctures associated with the CPU.  When the other CPU is resumed, those modifications can safely take place.

The three fixes are to handle cases in the SMP configuration where one CPU does need to make modifications to TCB and data structures on a task that could be running running on another CPU.  Those three cases are task_delete(), task_restart(), and execution of signal handles.  In all three cases the solutions is basically the same:  (1) Call sched_cpu_pause(tcb) to pause the CPU on which the task is running, (2) perform the necessary operations, then (3) call up_cpu_resume() to restart the paused CPU.