systemd: resolve executable path if it is relative

systemd: resolve executable path if it is relative

Backport a patch to fix the below systemd-analyze issue:
 # systemd-analyze --man=false verify /lib/systemd/system/initrd-cleanup.service
 initrd-cleanup.service: Command systemctl is not executable: No such file or directory

 # which systemctl
 /bin/systemctl

 # cat /lib/systemd/system/initrd-cleanup.service
 [snip]
 [Unit]
 Description=Cleaning Up and Shutting Down Daemons
 DefaultDependencies=no
 AssertPathExists=/etc/initrd-release
 OnFailure=emergency.target
 OnFailureJobMode=replace-irreversibly
 After=initrd-root-fs.target initrd-fs.target initrd.target

 [Service]
 Type=oneshot
 ExecStart=systemctl --no-block isolate initrd-switch-root.target

Signed-off-by: Mingli Yu <mingli.yu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>