update-kernel script prints warning even if extra firmware doesn't exist

update-kernel

Warning: extra firmware "" not found!
%<...

Install extra firmware only if modloopfw variable is not empty:

--- /sbin/update-kernel
+++ /tmp/update-kernel.new
@@ -293,14 +293,16 @@
 fi

 # install extra firmware files in modloop (i.e. not detected by modinfo)
-for _xfw in "$modloopfw"; do
-       if [ -f "$ROOT/lib/firmware/$_xfw" ]; then
-               install -pD "$ROOT/lib/firmware/$_xfw" \
-                       "$MODLOOP"/modules/firmware/"$_xfw"
-       else
-               echo "Warning: extra firmware \"$_xfw\" not found!"
-       fi
-done
+if [ -n "$modloopfw" ]; then
+       for _xfw in "$modloopfw"; do
+               if [ -f "$ROOT/lib/firmware/$_xfw" ]; then
+                       install -pD "$ROOT/lib/firmware/$_xfw" \
+                               "$MODLOOP"/modules/firmware/"$_xfw"
+               else
+                       echo "Warning: extra firmware \"$_xfw\" not found!"
+               fi
+       done
+fi

 # include bluetooth firmware in modloop
 if [ -e "$ROOT"/lib/modules/*/kernel/drivers/bluetooth/btbcm.ko ]; then

(from redmine: issue id 10526, created on 2019-05-31)