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
Use case: `mkbootimg` provides the `unpackbootimg` package. When
running `pmb.chroot.apk.install(args,"unpackbootimg")`, it was not
able to properly build the package.
Reproducing the error:
```
sudo rm ~/.local/var/pmbootstrap/packages/x86_64/mkbootimg*
pmbootstrap index
pmbootstrap --mirror-pmOS="" chroot --add=unpackbootimg
```
Or alternatively (simpler but less illustrative):
```
pmbootstrap build unpackbootimg --force
```
Closes#441. Adjust bootimg_analyze code:
* Install mkbootimg (which now provides unpackbootimg) instead of
unpackbootimg. In theory, pmbootstrap should recognize this
automatically, however right now it does not yet handle this case.
* The file names of the extracted files have changed.
Oliver Smith
Lucas Ramage
drebrez