package/f2fs-tools: furthe endianness fixes

Add appropriate checks in configure.ac.

Fixes a slew of autobuilder failures:
    http://autobuild.buildroot.org/results/cf6/cf6f5b6981694f74bfca45b3d04ff35e4226b162/
    http://autobuild.buildroot.org/results/b34/b346472e686af5b517b689217349eab21878bb7b/
    http://autobuild.buildroot.org/results/49a/49a6a2a88cac6e0899f271a5e65c78dd9eb2eab4/


    ...

Note: patches sent upstream, awaiting for reply...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>