phosh.service: Reduce boot flicker

Reduce flicker during boot by using tty1 and stopping
plymouth ourself.
5 jobs for boot-flicker in 19 minutes and 34 seconds (queued for 1 second)
Status Name Job ID Coverage
  Package
passed package-pureos-byzantium #366201
librem5

00:08:31

passed package-pureos-byzantium:arm64 #366202
librem5:arm64

00:19:34

 
  Test Package
passed autopkgtest-pureos-byzantium-package #366203
librem5

00:01:20

passed check-tarball #366205
librem5

00:00:53

passed lintian-pureos-byzantium-package #366204
librem5

00:00:54