SAM3/4: Fix GPIO pull-up/down code.

Enabling the pull-down resistor while the pull-up resistor is still enabled
is not possible. In this case, the write of PIO_PPDER for the relevant I/O
line is discarded. Likewise, enabling the pull-up resistor while the
pull-down resistor is still enabled is not possible. In this case, the
write of PIO_PUER for the relevant I/O line is discarded.

All SAM Ethernet Drivers: Add support so that the drivers can be built with CONFIG_NET_MULTIBUFFER=y

drivers/sensors: Adds support for the  Sensixs XEN1210 3D-boar.  This sensor is used on NANOSATC-BR2 a Brazillian CUBESAT project.

include/nuttx/modem:  Move all modem-related IOCTL commands to a common file to assure that they will be unique

Basic Support for STM32L476 MDK Board

The Motorola MDK is based off of an earlier version
of NuttX.  This only provides a basic nsh shell.

Signed-off-by: Jim Wylder <jwylder@motorola.com>

For STM32L4 parts, the higher number USART ports supported
varies.  Add the HAVE_USARTx definitions to the configuration
to allow enabling the higher numbered USART ports.

Signed-off-by: Jim Wylder <jwylder@motorola.com>

restore stm32l4 name

Register renames to allow stm32l4 usb device compilation

Upstream_to_greg