pmbootstrap init: kernel selection / remove linux-pmos-lts (#1363)

* As discussed in IRC/matrix, we're removing `linux-postmarketos-lts`
  for now. The kernel isn't used right now, and we save lots of
  maintenance effort with not updating it every week or so.
* new config option `"kernel"` with possible values:
  `"downstream", "mainline", "stable"` (downstream is always
  `linux-$devicename`)
* ask for the kernel during `pmbootstrap init` if the device package
  has kernel subpackages and install it in `_install.py`
* postmarketos-mkinitfs: display note instead of exit with error when
  the `deviceinfo_dtb` file is missing (because we expect it to be
  missing for downstream kernels)
* device-sony-amami:
  * add kernel subpackages for downstream, mainline
  * set `deviceinfo_dtb`
* device-qemu-amd64: add kernel subpackages for stable, lts, mainline
* test cases and test data for new functions
* test case that checks all aports for right usage of the feature:
  * don't mix specifying kernels in depends *and* subpackages
  * 1 kernel in depends is maximum
  * kernel subpackages must have a valid name
  * Test if devices packages reference at least one kernel
* Remove `_build_device_depends_note()` which informs the user that
  `--ignore-depends` can be used with device packages to avoid building
  the kernel. The idea was to make the transition easier after a change
  we did months ago, and now the kernel doesn't always get built before
  building the device package so it's not relevant anymore.
* 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.