echo: add a basic echo command

Useful for simple boot tests.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
md5sum: fix ugly spew when file doesn't exist

We'd get an ugly spew if the file doesn't exist. Replace it with
a much nicer message.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>