using AIX 'csum' command for generating MD5 checksums

md5 and md5sum are not available in AIX by default.

AIX provides csum for that purpose. Links to official technotes and documentation:
[How to get the MD5 checksum for a file: md5sum, digest, csum, fciv](http://www-01.ibm.com/support/docview.wss?uid=swg21496703)
[csum Command](http://pic.dhe.ibm.com/infocenter/aix/v7r1/topic/com.ibm.aix.cmds/doc/aixcmds1/csum.htm)