Fixed gconfig target.

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

6loWPAN: Add option to dump buffers.  Fix some issues with calculation of the IEEE802.15.4 header size.

STM32:add I2C3 SDA pin mapping for STM32F411

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

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.