kinect_upload_fw: support architectures that are not little-endian

Add an endian.c program which will be used at compile time to generate
an endian.h file with the information about the endianness of the
architecture. This way we do not depend on any particular build system
to retrieve this information (libfreenect knows the endianness with the
help of Cmake, for instance).

This is inspired from the rdesktop project[1] and looks more portable
than relying on the system endian.h as its use is relatively new and not
standardized yet, even though there are proposals in that direction[2].

Finally fn_le32() implementation is ripped off from libfreenect.

[1] http://www.rdesktop.org/archive/2001/msg00722.html
[2] http://austingroupbugs.net/view.php?id=162