libopkg: fix SHA256 calculation for big endian system

libopkg: fix SHA256 calculation for big endian system

The SHA256 implementation relied on WORDS_BIGENDIAN being defined for
big endian systems, but this was broken since the switch to cmake,
causing wrong checksums on big endian systems like ar71xx.

Fix this by adding a cmake check for endianess similar to the one done
by the original autotools.

Fixes: e6e6c5a21634 ("build: add cmake build system")
Acked-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>