Remote sstate-cache mirror & first kanto migration steps

[mikehaller]

PR also contains some minor fixes and beginning of removing k3s stuff to replace with Kanto-CM, as some of these fixes were necessary to make the build work.

Changing the mirror configuration and DevContainer setup to optionally use a remote sstate-cache and BitBake downloads mirror.

If the GitHub Repository (eg our developer forks) have the following secrets set, the cache will be mounted inside of the DevContainers, so that we can update the remote caches with updated sstate-cache files:

AZURE_MOUNT_POINT="azure-sstate-cache"
AZURE_STORAGE_ACCESS_KEY="<secret>"
AZURE_STORAGE_ACCOUNT="sdvyocto"
AZURE_STORAGE_ACCOUNT_CONTAINER="yocto-sstate-cache"

To use the mirrors, add the "kas/mirrors.yaml" to the kas invocation using the ":" notation:

kas build kas/.config-kirkstone.yaml:kas/mirrors.yaml

This improves the build times dramatically and may save up to 80% of the build time, as long as the remote sstate-caches are kept up to date.

[stlachev]

LGTM

[stlachev]

LGTM

[stlachev]

LGTM

[stlachev]

LGTM

[stlachev]

LGTM