Keep selinux label for firmware images same

ueventd reads and loads firmware from
/system/etc/firmware,/vendor/firmware,/firmware/image

This are labelled as system_file, vendor_file.
label this as firmware_file so that we do not have to
expose /system and /vendor partition.

Change-Id: I9e505b504fd78d028e28bc4d1f549e4cebeb77fc