The BSDs have their endianness changing functions in <sys/endian.h> and
define both _BYTE_ORDER as well as BYTE_ORDER (if strict POSIX conformance
isn't requested). Extend the header to compile, so it supports these
platforms as well.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>