clicker2-stm32: Adds SD card, automount, and syslog file support and fixes a few minor issues

* configs/clicker2-stm32: Moves defines for xbee from clicker2-stm32.h to stm32_xbee.h

* configs/clicker2-stm32: Adds support for uSD click boards and automount support

* configs/clicker2-stm32: Fixes minor guard clause

* clicker2-stm32: Bring-up automounter before MMCSD

* clicker2-stm32: MRF24J40 interrupt should only fire on falling edge.

* clicker2-stm32: Adds file syslog support for logging to file on SD card

Approved-by: Gregory Nutt <gnutt@nuttx.org>