Add channels.cfg file and update pmaports.cfg.

I have a bunch of pmbootstrap patches in the works, that will make the channel selection possible ("channels" branch in pmbootstrap). They depend on this MR, it would be great if this MR could get reviewed / merged soon.

channels.cfg: new file

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] * latest: Upcoming beta release (WIP, DO NOT USE!)
[20:46:22] Channel [edge]:

pmaports.cfg: add channel

Replace branch_pmos and branch_alpine keys (that were never used) with a new channel key. pmbootstrap will look up the branches related to a channel in channels.cfg of the master branch. While at it, modernize the file.

Closes: #519 (closed)

channels.cfg

+# Reference: https://postmarketos.org/channels.cfg
+[channels.cfg]
+recommended=edge
+
+[edge]
+description=Rolling release channel
+branch_pmaports=master
+branch_aports=master
+mirrordir_pmos=master
+mirrordir_alpine=edge
+
+[stable]
+description=Upcoming beta release (WIP, DO NOT USE!)
+branch_pmaports=release/20.04
+branch_aports=3.11-stable
+mirrordir_pmos=v20.04
+mirrordir_alpine=v3.11