Target: fix read_tree_values for empty dir

grep was existing with 1 when passed an empty directory, resulting in
TargetError being raised unless explicitly suppressed with
check_exit_code=False. This case is now fixed to correctly return an
empty dict without error.

read_tree_values bash function has also been optimized to not
unnecessarily run find in a subshell if the path passed to the function
does not exist.

devlib/bin/scripts/shutils.in

@@ -216,10 +216,13 @@ read_tree_values() {
     PATH=$1
     MAXDEPTH=$2
 
+    if [ ! -e $PATH ]; then
+        echo "ERROR: $PATH does not exist"
+        exit 1
+    fi
+
     PATHS=$($BUSYBOX find $PATH -follow -maxdepth $MAXDEPTH)
-    if [ ${#PATHS[@]} -eq 0 ]; then
-        echo "ERROR: '$1' does not exist"
-    else
+    if [ ${#PATHS[@]} -gt 1 ]; then
         $BUSYBOX grep -s '' $PATHS
     fi
 }

devlib/target.py

@@ -620,6 +620,8 @@ class Target(object):
                                     check_exit_code=check_exit_code)
         result = {}
         for entry in output.strip().split('\n'):
+            if not entry.strip():
+                continue
             path, value = entry.strip().split(':', 1)
             result[path] = value
         return result