QCBOR: Add xx_IsBufferNULL(); update docs; endianness options

- xx_IsBufferNULL() allows caller of QCBOREncode to know if
  encoding is just a size calculation or a real encoding.
- Large changes to improve documentation and style in the .h
  files.
- New #define config options for compiler / OS / CPU specific
  endianness conversion that can reduce code size slightly.
  No incompatible changes and as before no requirement to
  define config for endianness.

Change-Id: Iaa443e6ac39c536dd5ed3b6f1316fd18cb624832
Signed-off-by: Laurence Lundblade <lgl@securitytheory.com>
7 files changed