There are boot flow mockups on 
https://gitlab.com/postmarketOS/pmaports/-/issues/1356

My proposal:

1. Add an unix socket to pbsplash that is listened on for commands while 
it's displaying the splash screen in the initfs

2. Create a pbctl utility to send messages to that socket to update the 
screen

There would be 4 different states for the UI that can be switched to 
with pbctl:

* Display the loading screen dots with `pbctl --dots`

* Display a text message with `pbctl --text "Resizing the rootfs"`

* Display a progress bar with `pbctl --progress 42`

* Display a battery level with `pbctl --battery 
/sys/class/power_supply/bat1`

Additionally for switching to a different theme there should be a 
command to pick a different svg to render as "logo"

* pbctl --svg /usr/share/something/logo-error.svg

Possibly these should be able to be combined to have an atomic update to 
switch to the error theme and an error message in the same frame.

Doing this with pbctl and a socket means that the transitions between 
these states can be perfect without any blinks or restarting for a 
smoother boot experience.