pmbootstrap init: kernel selection

Created by: ollieparanoid

Example output

[17:12:27] Device [qemu-amd64]: sony-amami
[17:12:29] Which kernel do you want to use with your device?
[17:12:29] Upstream kernels (mainline, stable, lts) get security updates, but may have less working features than downstream kernels.
[17:12:29] Available kernels (2):
[17:12:29] * downstream: Display and wifi works (see device table for details)
[17:12:29] * mainline: For kernel development only (most features aren't working)
[17:12:29] Kernel [downstream]:

Details

• new config option "kernel" with possible values: "downstream", "mainline", "stable", "lts" (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

Closes #1263 (closed). When this is merged, I will update the device specific package wiki page.