Fixes#1882
Tests performed:
- During the .deb package installation, the firstboot secret
is shown correctly
- the firstboot secret is separated from the next prompt after
running `cat /var/lib/plinth/firstboot-wizard-secret`
- plinth web interface accepts the firstboot secret
Signed-off-by: Veiko Aasa <veiko17@disroot.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>
0df674dc20