install-update: Do not use find -printf

Busybox `find` does not implement `-printf`.  We have to use `awk` to
get the base name of the file from its path.

install-update.sh

@@ -20,7 +20,8 @@ set_default_boot() {
     _rc=0
     mkdir -p newroot || return
     mount -oro "$1" newroot || return
-    _kernel=$(find newroot/boot -name 'vmlinuz-*' -printf '%P\n' \
+    _kernel=$(find newroot/boot -name 'vmlinuz-*' \
+        | awk -F/ '{print $NF}' \
         | sort -V \
         | tail -n1
     )