cmd: md5sum: use hash_command

Drop old implementation and use hash_command() instead, as
how it's currently done for crc32 and sha1sum cmds.

Test:
=> md5sum 0x60000000 0x200
md5 for 60000000 ... 600001ff ==> e6bbbe95f5b41996f4a9b9af7bbd4050

Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>