buildman: Improve [make-flags] section parser to allow quoted strings

The parser responsible for the '[make-flags]' section in
the '.buildman' settings file is currently not able to
handle quoted strings, as given in the sample bellow:

[make-flags]
qemu_arm=HOSTCC="cc -isystem /add/include" HOSTLDFLAGS="-L/add/lib"

This patch replaces the simple string splitter based on the <space>
delimiter with a regex tokenizer that preserves spaces inside double
quoted strings.
Signed-off-by: NCristian Ciocaltea <cristian.ciocaltea@gmail.com>

tools/buildman/toolchain.py

@@ -435,9 +435,10 @@ class Toolchains:
         self._make_flags['target'] = board.target
         arg_str = self.ResolveReferences(self._make_flags,
                            self._make_flags.get(board.target, ''))
-        args = arg_str.split(' ')
+        args = re.findall("(?:\".*?\"|\S)+", arg_str)
         i = 0
         while i < len(args):
+            args[i] = args[i].replace('"', '')
             if not args[i]:
                 del args[i]
             else: