We are analyzing the `boot.img` with `file` before we send it to
`unpackbootimg`. File does not recognize all kinds of `boot.img` files,
which `unpackbootimg` can extract, so we need a way to skip this check.
Details:
* Add `-f` parameter, continues extraction with a warning if the file
seems to be invalid
* Tell the user that `-f` can be used if the `boot.img` is invalid and
it's not specified
* Consistent spelling of `boot.img` instead of `bootimg` in messages
Fixes#1608
Oliver Smith
drebrez