imgtool: allow address adjustment in output hex

Add a new flag `-x` (or `--hex_addr`) which adjusts the memory address
where this file has to be written to. This is useful when generating
upgrade images that will go to the secondary slot, in cases where the
user is not using mcumgr or some other delivery mechanism, and has to
manually adjust the addresses using `objcopy`.

Also when using hex files, image padding now only adds a segment with
the magic at the end instead of filling the whole output with `0xff`.

Signed-off-by: Fabio Utzig <utzig@apache.org>