Add sha256 ckecksums to okpg

Add sha256 ckecksums to okpg

Thanks to Camille Moncelier <moncelier@devlife.org>
http://groups.google.com/group/opkg-devel/browse_thread/thread/78a2eb328da0ef73?utoken=pV1Kli0AAADKDldt5ZXsDDLs9sWCpWZI0mClVcTs45ANzZ7C9NH-1YGBxa5Bow63PTuzFmQCb1c

Here is a patch which adds sha256 checksum checking to Opkg.
More Opkg patches will follow shortly (x509 and smime signature support,
libcurl client/server authentication)

I hope these patch will be useful and finds their ways into okpg

Camille Moncelier
http://devlife.org/

git-svn-id: http://opkg.googlecode.com/svn/trunk@220 e8e0d7a0-c8d9-11dd-a880-a1081c7ac358