tools/configure.sh:  Fix last commit.  If gmake is not present, then 'which gmake' spews a lot of annoying error messages when configuring.  Best to pipe stderr to /dev/null so that we are blessedly unaware of the failures.

Add clang support to stm32f746g-disco board

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

drivers/1wire: add PM hooks and unlink support to sht21 sensor (allowing hot-swappable I2C bus from 1-wire converter)

* drivers/1wire: add PM hook to 1wire bus

* drivers/1wire/ds28e17.c: i2c instance must be zeroed

* drivers/sht21: add unlink support

    Unlike most other unlink implementations in NuttX drivers, this one
    does not allow any I2C operations after unlink, making it possible to
    deallocate the I2C bus.

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

configs/stm32f4discovery/hciuart:  Disable MMC/SD support.  That made it difficult to re-use the configuration without the base board.  MMC/SD support is not needed in that configuration anyway.

arch/arm/sc/lpc16xx:  Extends commit 456a258c:  Freeing endpoints should be set the same allocations as when endpoint was allocated.

configs:  Review all implementations of board_lcd_initialize().  The NuttX convention is that all internal functions that return errors as an 'int' must return a negated errno value on failure and a non-negative value on success.  Most were right but eight of them had cloned logic that returns 1 on success and zero on failure... both of which are interpreted as success by the caller since they are non-negative.

include/nuttx/wireless:  Add support for the rest of the IOCTL options needed to support apps/wireless/bluetooth/btsak.  This is only the IOCTL command definitions.  The IOCTL command implementation is still missing.

arch/arm/src/nrf52:  Add watchdog timer drivers.  Includes significant updates from Alan Carvalho de Assis <acassis@gmail.com>

drivers/net: Add TAP (network bridge) mode to tun.c

In previous implementation, TAP mode did not handle a packet
correctly. Also, the driver did not set the link layer type and
could not assign the interface name. This PR changes fix such
issues and supports TAP mode correctly.

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

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

drivers/wireless/bluetooth:  Partition bt_uart.c adding bt_uart.h and bt_uart_generic.c.  This will now permit adding HCI UART drivers that have custom initialization requirements.. such as parts the need to download a configuration file to the HCI UART before they are usable.

configs/Kconfig:  Fix some breakage inflicted on the Kconfig file by the i.MX RT merge.  Fix some coding standard problems in hymini-stm32v board support.

wireless/bluetooth:  Add support for IOCTL commands to perform GATT discovery and obtain the results.