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 59 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Package
passed #366206
librem5
package-pureos-byzantium

00:08:26

passed #366207
librem5:arm64
package-pureos-byzantium:arm64

00:19:57

 
  Test Package
passed #366208
librem5
autopkgtest-pureos-byzantium-package

00:01:11

passed #366210
librem5
check-tarball

00:01:25

passed #366209
librem5
lintian-pureos-byzantium-package

00:00:47