### Only download APKINDEX for relevant architectures
We're downloading the APKINDEX files for all architectures supported by
postmarketOS currently (x86, x86_64, armhf, aarch64). Most of the time,
we only need it for the native and device arch, so this PR reduces the
downloaded files to what is really necessary.
### Intuitive pmbootstrap update logic
* pmb.helpers.repo.update():
* Default is updating all arches where the APKBUILD files exist
* Add existing_only parameter
* Return True when files have been downloaded
* Properly print which arches will be updated
* Print update reason only in verbose log
* Add and improve comments
* pmb.parse.arguments(), update action:
* Add --non-existing parameter
* Default for --arch is None (instead of arch.native)
* pmb.helpers.frontend.update():
* Inform about --non-existing if no APKBUILDs have been updated
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