shared: generator - correct path to systemd-fsck

In generated systemd-fsck-root.service. This would break if rootprefix
is not /usr/lib/systemd.

[tomegun: flesh out commit message]

Makefile.am

@@ -188,6 +188,7 @@ AM_CPPFLAGS = \
 	-DCATALOG_DATABASE=\"$(catalogstatedir)/database\" \
 	-DSYSTEMD_CGROUP_AGENT_PATH=\"$(rootlibexecdir)/systemd-cgroups-agent\" \
 	-DSYSTEMD_BINARY_PATH=\"$(rootlibexecdir)/systemd\" \
+	-DSYSTEMD_FSCK_PATH=\"$(rootlibexecdir)/systemd-fsck\" \
 	-DSYSTEMD_SHUTDOWN_BINARY_PATH=\"$(rootlibexecdir)/systemd-shutdown\" \
 	-DSYSTEMD_SLEEP_BINARY_PATH=\"$(rootlibexecdir)/systemd-sleep\" \
 	-DSYSTEMCTL_BINARY_PATH=\"$(rootbindir)/systemctl\" \

src/shared/generator.c

@@ -61,7 +61,7 @@ static int write_fsck_sysroot_service(const char *dir, const char *what) {
                 "[Service]\n"
                 "Type=oneshot\n"
                 "RemainAfterExit=yes\n"
-                "ExecStart=/usr/lib/systemd/systemd-fsck %2$s\n"
+                "ExecStart=" SYSTEMD_FSCK_PATH " %2$s\n"
                 "TimeoutSec=0\n",
                 program_invocation_short_name,
                 what,