buildman: Extract tag parsing from parse() into _parse_tag()

Move the handling of tag lines (name:, desc:, fragment:, targets:) into
a separate method to reduce complexity of parse()

Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

tools/buildman/boards.py

@@ -1114,6 +1114,22 @@ class ExtendedParser:
                             f'{fname}:{linenum}: Invalid target regex')
                     self.targets.append(['regex', line.strip()])
             else:
+                self._parse_tag(fname, linenum, line)
+
+        self.finish()
+        return self.extended
+
+    def _parse_tag(self, fname, linenum, line):
+        """Parse a tag line (one not starting with a space)
+
+        Args:
+            fname (str): Filename (for error messages)
+            linenum (int): Line number (for error messages)
+            line (str): Line to parse
+
+        Raises:
+            ValueError: Invalid syntax
+        """
         pair = line.split(':')
         if len(pair) != 2:
             raise ValueError(f'{fname}:{linenum}: Invalid tag')
@@ -1132,6 +1148,3 @@ class ExtendedParser:
             self.in_targets = True
         else:
             raise ValueError(f"{fname}:{linenum}: Unknown tag '{tag}'")
-
-        self.finish()
-        return self.extended