Add endian conversion functions needed for network protocols.
Currently Bluetooth needs sys_le16_to_cpu and sys_cpu_to_le16
to implement endian safe stack. Note that the OS is not guaranteed
to be endian safe generally.
Change-Id: If7faf9568e0f2b1817139a76d9b00888d5b7d76c
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>