buildscript: Split removal of chipset and subtarget

Some OpenWrt targets do not have subtargets. The filename only contains
the chipset in that case. Split the removal of chipset and subtarget
into multiple expressions, so the removal of the chipset works on
targets without subtargets as well.

Fixes: #187

Signed-off-by: Fabian Bläse <fabian@blaese.de>

buildscript

@@ -290,7 +290,8 @@ cp_firmware() {
             filename_build=${f##*/}
             filename_build=${filename_build//openwrt/fff-${version}}
             filename_build=${filename_build//squashfs-/}
-            filename_build=${filename_build//${chipset}-${subtarget}-/}
+            filename_build=${filename_build//${chipset}-/}
+            filename_build=${filename_build//${subtarget}-/}
             cp "$f" "$imagedestpath/$filename_build"
         done
     done