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.
Signed-off-by: Ross Burton <ross.burton@intel.com>
