Ignore indented .config assignments.

Just noticed that the C implementation does. Haven't run into it in the
wild.

For example, the assignment to B below is ignored:

CONFIG_A=y
 CONFIG_B=y

1 files changed, 1 insertions, 1 deletions

diff --git a/kconfiglib.py b/kconfiglib.py
index ac907f0..8fa7a36 100644
--- a/kconfiglib.py
+++ b/kconfiglib.py
@@ -290,7 +290,7 @@ class Config(object):
             if line is None:
                 return
 
-            line = line.strip()
+            line = line.rstrip()
 
             set_match = set_re_match(line)
             if set_match: