Displays all historical memory allocations and deallocations.
   The data is collected in a ring buffer that is sized via
   the sysconfig inet attribute: ovms_memhist_npag, which represents
   the number of pages from nonpaged pool that will be allocated to
   collect the data.  To enable tracing, set the inet attribute
   ovms_memhist_enable=1.

   For example:

   1) View the current state of tracing:

        $ sysconfig -q inet ovms_memhist_enable ovms_memhist_npag
        inet:
        ovms_memhist_enable = 0
        ovms_memhist_npag = 2

      This shows that tracing is currently disabled.  By default,
      two pages of trace data will be recorded.

   2) Enable tracing

        $ sysconfig -r inet ovms_memhist_enable=1
        SDA> tcpip show memory /history

      If tracing is stopped, the data is still available.

        $ sysconfig -r inet ovms_memhist_enable=0
        SDA> tcpip show memory /history

   3) Free the nonpaged pool allocated to the ring buffer

        $ sysconfig -r inet ovms_memhist_enable=1
        $ sysconfig -r inet ovms_memhist_npag=0
        SDA> tcpip show memory /history
           ... no data is displayed ...