update_font_cache: only scan system font directories

By default fc-cache will scan both system and user directories, which means it
attempts to scan $HOME/.fonts.  As this is the build host's idea of $HOME this
generally doesn't exist, and causes fc-cache to exit with a failure.

Solve this by passing --system-only so that fc-cache will only scan system
directories, as is appropriate for a rootfs-time invocation.

(From OE-Core rev: 541315d6c56df6448f64c262f99d43d5c1e9400b)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/postinst-intercepts/update_font_cache

@@ -1,5 +1,5 @@
 #!/bin/sh
 
 PSEUDO_UNLOAD=1 qemuwrapper -L $D -E LD_LIBRARY_PATH=$D/${libdir}:$D/${base_libdir} \
-					-E ${fontconfigcacheenv} $D${bindir}/fc-cache --sysroot=$D ${fontconfigcacheparams}
+					-E ${fontconfigcacheenv} $D${bindir}/fc-cache --sysroot=$D --system-only ${fontconfigcacheparams}
 chown -R root:root $D${fontconfigcachedir}