We need to add a delay between setting and clearing the endpoint reset
bit in SAM_UDP_RSTEP. Without the delay the USB controller will (may?)
not reset the endpoint.

If the endpoint is not being reset, the Data Toggle (DTGLE) bit will
not to be cleared which will cause the next transaction to fail if
DTGLE is 1. If that happens the host will time-out and reset the bus.

Adding this delay may also fix the USBMSC_STALL_RACEWAR in
usbmsc_scsi.c, however this has not been verified yet.

Most interrupt handling logic interacts with tasks via standard mechanism such as sem_post, sigqueue, mq_send, etc.  This all call enter_critical_section and are assumed to be safe in the SMP case.

But certain logic interacts with tasks in different ways.  The only one that comes to mind are wdogs.  There is a tasking interface that to manipulate wdogs, and a different interface in the timer interrupt handling logic to manage wdog expirations.

In the normal case, this is fine.  Since the tasking level code calls enter_critical_section, interrupts are disabled an no conflicts can occur.  But that may not be the case in the SMP case.  Most architectures do not permit disabling interrupts on other CPUs so enter_critical_section must work differently:  Locks are required to protect code.

So this change adds locking (via enter_critical section) to wdog expiration logic for the the case if the SMP configuration.

drivers/net:  Add option to use low-priority work queue to all drivers in drivers/net.  Not yet added to all architecture-specific network drivers.

Timerfixes

SMP: irq_csection() has a bad assumption.  It assumed that the state of certain variables.  That was true on entry into the interrupt handler, but might change to the execution of logic within the interrupt handler.

apps/examples/timer:  Restore the timer example, but adapt the interface to use the new signal logic from Sebastien, Lorquet.  Totally untested and probably does not work!

All timer lower half drivers.  Port Sebastien's changes to all all other implementations of the timer lower half.  Very many just and untested.  Expect some problems.

timer driver: Use signal to notify of timer expiration. Add generic argument so that there can be additional usage.

Timer driver:  Add hooks to support signal notification of timer expiration.  Commented out because invasive interface changes would also be required to complete the implementation.

drivers/timer:  Remove the TIOC_SETHANDLER IOCTL call.  This calls directly from the timer driver into application code.  That is non-standard, non-portable, and cannot be supported

Add some experimental changes to enter/leave_critical_section to deal with the case where interrupts are disabled only on the local CPU

If SMP is enabled, if any interrupt handler calls enter_critical_section(), it should take the spinlock.