imgtool: fix passing --erased-val with 0xff value

The `--erased-val` (or `-R`) option was doing conversion of base 10, so
the only value that was acceptable was 0. Although not passing this
option would result in the default `0xff` being used, this fixes the
issue by allowing Python to auto-detects the base from string.

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