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.
