uptime: allow the optional limiting of kernel runtime

Introduce the ability to limit the limit the uptime of a kernel.
When enabled, these options set a maximum uptime on the kernel, and
(optionally) trigger a clean reboot at expiration.

This functionality may appear to be very close to the softdog watchdog
implementation. It is. But can't be the softdog for several reasons:

  - The soft watchdog should be available while this functionality is active
  - The duration range is different between this and the softdog. The
    timeout available here is potentially quite a bit longer.
  - At expiration, there are different expiration requirements and actions.
  - This functionality is specific to a particular use case and should
    not impact mainline functionality

To cleanly restart the kernel after one minute of uptime, the following
config items would be required:

  CONFIG_UPTIME_LIMITED_KERNEL=y
  CONFIG_UPTIME_LIMIT_DURATION=1
  CONFIG_UPTIME_LIMIT_KERNEL_REBOOT=y

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>