bluez5_utils: fix build with pkgconf 1.5.3

bluez5_utils: fix build with pkgconf 1.5.3

Since the bump to 1.5.3, pkgconf prepends the sysroot to all absolute
paths found in the .pc file. This is correct when the paths refer to
something in STAGING_DIR (e.g. libdir, includedir), but not when it
refers to something used for the target.

bluez5_utils uses the sysconfdir variable from dbus-1.pc to decide where
to install things. Since DESTDIR is prepended to the install
destination, this will end up in the wrong location.

Until a better solution is found in pkgconf, pass the sysconfdir to use
explicitly instead of relying on dbus-1.pc.

Fixes:
 - http://autobuild.buildroot.org/results/498e7e368fbb3d69c2f311e3e5bc213e507b8023

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>