imgtool: fix header padding value check

Commit 9117fded added support for
supplying an `--erased-val`, which defaults to 0xff. When padding a
header, the checks were still validating against 0, which is the
value used for padding by the Zephyr build system. Now when the
user passes `--pad-header` this check is skipped (we already know
the contents).

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