The Linux Kernel defines only one of __LITTLE_ENDIAN and
__BIG_ENDIAN. Endianess can then be tested with #ifdef __xx_ENDIAN. Userspace
always defined both __LITTLE_ENDIAN and __BIG_ENDIAN and byteorder can then
be tested with #if __BYTE_ORDER == __xx_ENDIAN.

As we tend to use a lot of Kernel code in barebox we switch to use the kernel
way of determing the byte order.

As this always causes a lot of confusion add a check to include/common.h to
make sure only one of __LITTLE_ENDIAN and __BIG_ENDIAN is defined.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>