imgtool: skip erased values in trailer generation

When generating an image with trailer (--pad or --confirm), the
imgtool also adds the erased values into that area. This is not
wanted for flash drivers which use ECC and treat writing erased
values as a real write action, which cannot be overwritten anymore.
See issue #1288

Signed-off-by: Wouter Cappelle <wouter.cappelle@crodeon.com>