Define kdump_byte_order_t

Use own type, so the public header file doesn't depend on <endian.h>
if the endianness is exposed to library users.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
Add API to get dump file byte order

Provide an API to get the byte order.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>