update_engine: make it accept LDFLAGS

BUG=None
TEST=It accepts LDFLAGS now

Change-Id: Ibfd08e72db9b56d580fd474169f51b5124b5cdd1
Reviewed-on: https://chromium-review.googlesource.com/168047
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>

SConstruct

diff --git a/SConstruct b/SConstruct
index 215d4b1..3ce85b7 100644
--- a/SConstruct
+++ b/SConstruct
@@ -162,7 +162,7 @@
   value = os.environ.get(key)
   if value != None:
     env[key] = value
-for key in Split('CFLAGS CCFLAGS CPPPATH LIBPATH'):
+for key in Split('CFLAGS CCFLAGS LDFLAGS CPPPATH LIBPATH'):
   value = os.environ.get(key)
   if value != None:
     env[key] = Split(value)
@@ -172,6 +172,7 @@
     env['ENV'][key] = os.environ[key]
 
 
+env['LINKFLAGS'] = env['LDFLAGS']
 env['CCFLAGS'] = ' '.join("""-g
                              -fno-exceptions
                              -fno-strict-aliasing