systemd-compat-units.bb: fix postinstall script

This fixes an issue running "opkg upgrade" on a system with systemd
(and when there is an update to "systemd-compat-units",
for example between yocto 4.2.2 and 4.2.3):

//var/lib/opkg/info/systemd-compat-units.postinst: cd: line 3: can't cd to /etc/init.d: No such file or directory

The existence of /etc/init.d is now tested
without causing an error if doesn't exist.

Fixes [YOCTO #15292]

(From OE-Core rev: d114814fa2628cfea2769d65a26514b76e61a0fa)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 0330331a1386fd2a34b410a7f62b29bfc8dc23c4)
Signed-off-by: Steve Sakoman <steve@sakoman.com>

meta/recipes-core/systemd/systemd-compat-units.bb

@@ -27,7 +27,8 @@ SYSTEMD_DISABLED_SYSV_SERVICES = " \
 
 pkg_postinst:${PN} () {
 
-	cd $D${sysconfdir}/init.d  ||  exit 0
+	test -d $D${sysconfdir}/init.d  ||  exit 0
+	cd $D${sysconfdir}/init.d
 
 	echo "Disabling the following sysv scripts: "