STM32:  Move STM32F42xxx RCC definitions out of stm32f40xxx_rcc.h into their own stm32f42xxx_rcc.h header file

STM32:  Move STM32F42xxx DMA definitions out of stm32f40xxx_dma.h into their own stm32f42xxx_dma.h header file

STM32:  Move STM32F42xxx pinmap definitions out of stm32f40xxx_pinmap.h into their own stm32f42xxx pinmap.h header file

SSD1306:  Rename ssd1306_helpers.h to ssd1306.h.  Move all SSD1306 driver definitions from ssd1306_base.c to ssd1306.h

Modify the SSD1306 LCD driver to support either the SPI or I2C interface.  From Alan Carvalho de Assis,

NETDB: Make sure that we have a valid DNS server address before attempting DNS queuries; Provide an optional default DNS server address in the configuration

This completes all interface level support for IPv6 DNS.  Still some missing logic at the lowest protocol levels

netdb: Modifications to DNS client logic and to dns_get/setserver() interfaces to support DNS clients at IPv6 addresses (still no support for resolution to IPv6 addresses).

- Rename include/nuttx/net/dnsclient.h to dns.h
- Move internal DNS prototypes from dns.h to libc/netdb/lib_dns.h
- Global standard DNS definitions from libc/netdb/dns_soccket.c to dns.h
- Eliminate dns_gethostip().  This is now an internal part of gethostbyname()
- Eliminate interface dns_whois().  Not needed in this new context.

DNS: Remove some unused, non-functional DNS functions; remove the annoying _sock at the end of some DNS functions names, it is no longer needed to distinguish the functions

samdl/sam_usb.h: fix USBDEV_CTRLB_SPDCONF defines Datasheet 03-2015 and 06-2015 is wrong. These defines are correct verified with analyser and also same values are used on Atmel xdk-asf-3.24.2 sources.  From Janne Rosberg.