main/postmarketos-base: enable zram-init service on upgrade (MR 4598)

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.

Update by Oliver: add a comment to mention the deviceinfo variable

[ci:skip-build]: already built successfully in CI

main/postmarketos-base/APKBUILD

@@ -1,6 +1,6 @@
 pkgname=postmarketos-base
 pkgver=29
-pkgrel=0
+pkgrel=1
 pkgdesc="Meta package for minimal postmarketOS base"
 url="https://postmarketos.org"
 arch="noarch"

main/postmarketos-base/postmarketos-base.post-upgrade

@@ -23,4 +23,7 @@ ln -sf "$localtime" /etc/localtime
 rm -rf /etc/zoneinfo
 [ "$restart" = "yes" ] && service -qq openrc-settingsd restart
 
+# To turn zram-init into a no-op, set: deviceinfo_zram_swap_pct="0"
+rc-update -q add zram-init default
+
 exit 0