Include isomd5sum in GRMLBASE and support implantisomd5 within grml-live

[mika]

isomd5sum provides two binaries: implantisomd5 to embed a checksum into an ISO, and checkisomd5 which then checks the MD5 checksum which was implanted by implantisomd5.

Its usage is as simple as:

implantisomd5 grml.iso

and:

checkisomd5 /dev/sr0

A customer of mine is using this and this seems to be useful and worth consideration inclusion and integration in grml-live.