scripts: fix sloppy/unpredictable cmd && this || that statements

- be a bit more POSIX to avoid any potential issues when full shell stdout/err are redirected
- actual logic chains remain unchanged

scripts/boot_patch.sh

@@ -32,7 +32,14 @@
 # Pure bash dirname implementation
 getdir() {
   case "$1" in
-    */*) dir=${1%/*}; [ -z $dir ] && echo "/" || echo $dir ;;
+    */*)
+      dir=${1%/*}
+      if [ -z $dir ]; then
+        echo "/"
+      else
+        echo $dir
+      fi
+    ;;
     *) echo "." ;;
   esac
 }