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.

QSPI interrupt driven mode is now implemented

Backout 3f731241.  It is a good idea, but has some unpleasant side effect... like SYSLOG and dbg() no longer work

Add handshake to coordintate with connection of VNC client.  fb_initialize() will not return until the connection is established

basic support for QSPI in STM32L4; verified via 'examples/media'

add MTD support for Micron N25Qxxx family of QSPI flash