Add systemd service to delay boot

Rollback tests need to reset the DUT before boot is completed.
This commit add a service delaying start of AUM and user logging, and
displaying a message allowing a tester to reset the board.
This service should be run before systemd-bless-boot.service which rename
the EFI loader entry discarding the bootcount.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>