wireless/ieee802154: Add initialization logic for loopback driver; configs/sim: Add configuration for testing 6loWPAN; net/sixlowpan: Fix for compilation with debug output enabled.

drivers/sensors: Add driver for ST LIS2DH accelerometer.  From Timo Voutilainen <time.voutilainen@haltian.com> et al.

6loWPAN: Change ordering of some operations so that the IOB is available at the time that headr compression is perfomed.

Networking: Fix bad macro logic of recent commit.  Macros were not being evaluating in the order and time that I thought.

Upstream stm32f7 serial fixes

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

  1) Revert the inherited dma bug from the stm32
     see https://bitbucket.org/nuttx/nuttx/commits/df9ae3c13fc2fff2c21ebdb098c520b11f43280d
     for details.

  2) Most all CR1-CR3 settings can not be configured while UE
     is true. Threfore we make all operation atomic and disable
     UE and restore it's originalstate on exit.

net/:  Permit net/neighbor to build when IPv6 is defined, but not Ethernet.  Needs more work to support 6loWPAN.  Also included, some minor configuration updates for 6loWPAN.