misc/byteorder.h: Add helpers to get & put unaligned integers

These helpers provide a way to read and write data buffers containing
multi-byte integers with a specific endianness. They are particularly
convenient for network protocol encoding and decoding.

Change-Id: I09ad125a1bbc4510b97a1d75c83cb70d893e22e8
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>