net/: Finish IEEE802154->6LOWPAN renaming; remove PF_IEEE804154; Move 6LoWPAN menu to inside Internet Protocol Menu

drivers/wireless/ieee802154/Make.defs
    Changes to the build infrastructure to include the mrf24j40 driver

include/nuttx/spi/spi.h
    Define a IEEE SPI device

include/nuttx/wireless/ieee802154/ieee802154_radio.h
    This file defines the API of a IEEE 802.15.4 radio device, to be used by the future common MAC 802.15.4 layer

drivers/wireless/ieee802154/mrf24j40.c
drivers/wireless/ieee802154/mrf24j40.h
    The main MRF24J40 IEEE 802.15.4 radio driver

include/nuttx/wireless/ieee802154/mrf24j40.h
    This file allows instanciation of the MRF24j40 driver from board configurations

Move drivers/ieee802154 to drivers/wireless/ieee802154; rename include/nuttx/net ieee802154 to 6lowpan.h

sched: Recent change of ASSERT to DEBUGASSERT causes a crash when debug is diabled because there is a critical function call within the assertion

FAT: Add a new configuration option to decouple the logic that retries the direct transfer from the logic that enables DMA memory allocators.

FAT: Add an option to force all transfers to be performed indirectly through the FAT file system's internal sector buffers