ueventd: let scripts provide firmware directories

Since some vendors will have firmware in mount points in
/mnt/vendor/..., we extend the ueventd script language to allow
specifying the firmware directories.

Also, move the existing 4 directories to ueventd.rc as a primary user
of this mechanism.

Bug: 111337229
Test: boot sailfish; firmwares load
Change-Id: I0854b0b786ad761e40d2332312c637610432fce2
diff --git a/rootdir/ueventd.rc b/rootdir/ueventd.rc
index b03d83b..2f85dec 100644
--- a/rootdir/ueventd.rc
+++ b/rootdir/ueventd.rc
@@ -1,3 +1,5 @@
+firmware_directories /etc/firmware/ /odm/firmware/ /vendor/firmware/ /firmware/image/
+
 subsystem adf
     devname uevent_devname