yocto-bsp: use rstrip() for assignment lines

strip() isn't necessary and causes unintended formatting changes in
the output; rstrip() remove the trailing newlines as intended while
leaving indenting whitespace intact.

Signed-off-by: Tom Zanussi <tom.zanussi@intel.com>

scripts/lib/bsp/engine.py

@@ -668,7 +668,7 @@ class SubstrateBase(object):
         """
         Expand all tags in a line.
         """
-        expanded_line = AssignmentLine(line.strip())
+        expanded_line = AssignmentLine(line.rstrip())
 
         while start != -1:
             end = line.find(CLOSE_TAG, start)