kernel_flavors_installed(): add autoinstall=True

* pmb/chroot/other.py:
  * Add autoinstall=True to kernel_flavors_installed(). When the flag
    is set, the function makes sure that at least one kernel for the
    device is installed.
  * Remove kernel_flavor_autodetect() function, wherever it was used,
    it has been replaced with kernel_flavors_installed()[0].
* pmb.helpers.frontend.py: remove code to install at least one kernel,
  kernel_flavors_installed() takes care of that now.