binman: Don't generate an error in 'text' entry constructor

It is not good practice to raise an exception in a constructor. In this
case the 'text' entry may not actually be used, if -i is used to filter
out the images that get built.

Move the exception to where the data is actually used.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/tools/binman/etype/text.py b/tools/binman/etype/text.py
index 6e99819..c4aa510 100644
--- a/tools/binman/etype/text.py
+++ b/tools/binman/etype/text.py
@@ -51,10 +51,10 @@
         self.text_label, = self.GetEntryArgsOrProps(
             [EntryArg('text-label', str)])
         self.value, = self.GetEntryArgsOrProps([EntryArg(self.text_label, str)])
+
+    def ObtainContents(self):
         if not self.value:
             self.Raise("No value provided for text label '%s'" %
                        self.text_label)
-
-    def ObtainContents(self):
         self.SetContents(self.value)
         return True