imgtool: initial sanity test

An initial sanity test for imgtool is added, checks
different commands for key operations (keygen, getpriv,
getpub and getpubhash).

Also very basic test for sign / verify is added.

Some tests are disabled (marked as 'xfail') due to
the missing implementation.

Signed-off-by: Denis Mingulov <denis@mingulov.com>