add configuration options to allow SRAM2 to be used for heap, or not at all, and to zero-init it on OS start, or not at all.

QSPI DMA support in STM32L4, and DMA fixes...

fixed missing DMA peripheral selection and some header defines, updated various comments to be accurate

add QSPI memory mapped mode support.  tested.  QSPI may enter and exit memory mapped mode; while in effect, other operations (e.g. command, memory) will fail with -EBUSY.

add QSPI memory mapped mode support.  tested.  QSPI may enter and exit memory mapped mode; while in effect, other operations (e.g. command, memory) will fail with -EBUSY.

VNC: Ignore client framebuffer updates if nothing has changed (we can does this because client requests incremental updates

Fixed const warning