main/postmarketos-base: enable zram-init service on upgrade

The post-upgrade script in pmos-base isn't symlinked to the post-install script, so zram-init wasn't being enabled on systems that upgraded to the pmos-base version that intro'd this feature. I think this should have been enabled by default on upgraded systems. There's a deviceinfo toggle for it, so users who won't want to use this can set that var to disable it, in that case having the service enabled is basically a no-op.

(cc @pabloyoyoista)