package/dbus: fix group and mode for the launcher

The D-Bus installation process installs dbus-daemon-launch-helper as
follows:

  chown root:$(DBUS_USER) $(DESTDIR)$(libexecdir)/dbus-daemon-launch-helper$(EXEEXT); \
  chmod 4750 $(DESTDIR)$(libexecdir)/dbus-daemon-launch-helper$(EXEEXT); \

And when the installation does not take place as root (like is the
case in the context of Buildroot), it prints:

  echo "Not installing $(DESTDIR)$(libexecdir)/dbus-daemon-launch-helper binary setuid!"; \
  echo "You'll need to manually set permissions to root:$(DBUS_USER) and permissions 4750"; \

So let's adjust the installation logic of dbus-daemon-launch-helper to
match these requirements.

Signed-off-by: Norbert Lange <nolange79@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 7ac245a0cb76f270b28860936245ca1abba0804a)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/dbus/dbus.mk

@@ -11,7 +11,7 @@ DBUS_LICENSE_FILES = COPYING
 DBUS_INSTALL_STAGING = YES
 
 define DBUS_PERMISSIONS
-	/usr/libexec/dbus-daemon-launch-helper f 4755 0 0 - - - - -
+	/usr/libexec/dbus-daemon-launch-helper f 4750 0 dbus - - - - -
 endef
 
 define DBUS_USERS