target: Remove use of ls

Using "ls" in scripts is highly discouraged:
http://mywiki.wooledge.org/ParsingLs

devlib/target.py

@@ -168,9 +168,10 @@ def number_of_cpus(self):
     @memoized
     def number_of_nodes(self):
         num_nodes = 0
-        nodere = re.compile(r'^\s*node\d+\s*$')
-        output = self.execute('ls /sys/devices/system/node', as_root=self.is_rooted)
-        for entry in output.split():
+        nodere = re.compile(r'^\./node\d+\s*$')
+        cmd = 'cd /sys/devices/system/node && {busybox} find . -maxdepth 1'.format(busybox=quote(self.busybox))
+        output = self.execute(cmd, as_root=self.is_rooted)
+        for entry in output.splitlines():
             if nodere.match(entry):
                 num_nodes += 1
         return num_nodes