lvm2: install initscript or systemd unit service conditionally

Issue: LIN8-854

install systemd unit files and modify them only when DISTRO_FEATURE
systemd is enabled, otherwise do_install failed due to no
blk-availability.service file
   sed: can't read tmp/work/armv5e-wrs-linux-gnueabi/lvm2/2.02.125-r0/image/lib/systemd/system/blk-availability.service: No such file or directory

install the initscript when init is not systemd

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>