- implement flash driver
   - see arch/arm/src/stm32/stm32_flash.c and related
     - both banks could be mapped with care taken not to erase the active code.
       - perhaps the MPU could block code corruption?

   - once implemented, the free() cmd replacement can show flash information.

 - Seen crashes when running serial ports below 921600

    - COM34 (UART1, ttyS1) runs the smoothest with 'serialblaster' test...
    the other two (UART0, USART1) seem to stutter...

   - created a cpuhog test. Each instance takes a semaphore, burns cpu for 6ms and then releases
     the sem to exercises semaphores passing and a busy cpu. The first two instances become a
     producer/consumer pair using a pipe (FIFO) to exercise the fileio data passing.
       -> no crashes with 8 instances running. (No MMU, No BASEPRI)
       -> no crashes with 8 instances running. (MMU + BASEPRI)
          -> not however that the serial ports are very finicky with BASEPRI enabled!
            This setup sends single charactes for each produced or consumed block,
            and doesn't seem to have any issues:
              NuttShell (NSH)
              nsh> cpuhog > /dev/ttyS1 &
              cpuhog [5:50]
              nsh> cpuhog > /dev/ttyS2 &
              cpuhog [7:50]
              nsh> cpuhog &
              cpuhog [8:50]
              nsh> cpuhog 2
              cpuhog &
              cpuhog [9:50]
              nsh> cpuhog 3
              cpuhog &
              cpuhog [10:50]
              nsh> cpuhog 4
              cpuhog &
              cpuhog [11:50]
              nsh> cpuhog 5

              nsh> cpuhog &
              cpuhog [12:50]
              nsh> cpuhog 6

              nsh> cpuhog &
              cpuhog [13:50]
              nsh> cpuhog 7

 - USB serial not quite stable when pushing lots of data

 - UARTs don't use DMA. We may need this for high baudrates.
   - Inbound hardware flow control requires dma.

Current Test:

40mA:   120Mhz (240 MHz PLL) with SDCard mounted, UART0+UART1+USART1 enabled, USB Device availible (but nothing plugged in)
37.8mA: 120Mhz (120 MHz PLL) with SDCard mounted, UART0+UART1+USART1 enabled
16.9mA:  "" with WFI added to up_idle()