STM32L4 DAC: port from STM32.  Note that this does not address the somewhat confusing relation between STM32L4_NDACS and DAC2 config macros that comes from original STM32 code.

readme: more typo fixes

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

fix ESP32 gpio enable reg and default UART pin.

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

STM32F7: Some STM32F7 builds failed in build testing due to undefined STM32_SRAM1_BASE.  I think that is because stm32_allocateheap.c was not including chip/stm32_memorymap.h

C++: Compilation with recent C++ compiler needs an overloaded delete[] operator that includes a size_t size argument.

ARM: The older ARM7 and ARM9 configurations were determining CFLAGS based on the GCC version 4.x.x or not.  That needx to be extended for 5.x.x and 6.x.x which also behave like 4.x.x.

C++: Compilation with recent C++ compiler needs an overloaded delete operator that includes a size_t size argument.

procfs routing table:  Don't print the IPv4 routing table header if the table is empty.  Fix a couple of places where CONFIG_NET_IPv4 was used wehre IPv6 was intended (not noticed now because I am testing with both enabled).

cwchar:Use CONFIG_LIBC_WCHAR to only export the wc/mb functions

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

   When a build does not want to use wide or multibyte char
   CONFIG_LIBC_WCHAR is not set. Therefore we should to only
   export the wc/mb functions when defined.

   Regardless of the stat of CONFIG_LIBC_WCHAR the non mb/wc
   definitions such as mbstate_t, wint_t, wctype_t need to be
   exported.