treewide: Use CONFIG_*_ENDIAN instead of __BYTE_ORDER__

In order to avoid using multiple sources of truth for the platfom's
endianness, convert the in-tree code to use the (BIG|LITTLE)_ENDIAN
Kconfig variables exclusively, instead of the compiler's
__BYTE_ORDER__.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>