systemd-compat-units: move list of disabled services to variable, fix hwclock.sh and output

* variable allows to add items in .bbappend without overwritting whole
  postinst
* hwclock.service should be used to blacklist SYSV hwclock.sh (not hwclock.sh.service)
* show only newly disabled SYSV scripts

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>