Update resign-image command help text. - Removed <target> as an option. - Fixed typo and added a note that if signing key is not specified, the current signing key in the image is stripped.
Update resign-image command help text.
- Removed <target> as an option.
- Fixed typo and added a note that if signing key is not specified, the current signing key in the image is stripped.

apache/incubator-mynewt-newt,apache/incubator-mynewt-newt