x86_64 packages have been built, let's switch:
http://dl-cdn.alpinelinux.org/alpine/v3.12/
There doesn't seem to be a branch on aports.git yet, but it should
appear soon. So it's master for now.
Closes: #582
Let's add a nice file that defines the available release channels.
Upcoming pmbootstrap patches will make use of this in 'pmbootstrap init'
for example:
[20:46:22] Choose the postmarketOS release channel.
[20:46:22] Available (2):
[20:46:22] * edge: Rolling release channel
[20:46:22] * stable: Upcoming beta release (WIP, DO NOT USE!)
[20:46:22] Channel [edge]:
Closes: #519
