images/build_images.py: support ALL

The docs and build_images.py claims to support ALL but it doesn't
actually work.  This change makes it work.

BUG=None
BRANCH=None
TEST=Verified that "make" now actually works rather than fails.

Change-Id: Ia2163742b98ce6b435135fe2e8228c5806f4f7d1
Signed-off-by: Matt Delco <delco@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1508972
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Reviewed-by: Daisuke Nojiri <dnojiri@chromium.org>
diff --git a/images/build_images.py b/images/build_images.py
index 2165152..7fbee02 100755
--- a/images/build_images.py
+++ b/images/build_images.py
@@ -10,8 +10,8 @@
 Usage:
   ./build_images.py <board-names>
 
-  Use 'ALL' in <board-names> if you want to build every boards defined in
-  configuration file.
+  Use 'ALL' in <board-names> (or don't specify any names) if you want to build
+  every boards defined in configuration file.
 '''
 
 import copy
@@ -398,7 +398,7 @@
   configs = load_boards_config(BOARDS_CONFIG)
 
   targets = args
-  if not targets:
+  if not targets or targets == ['ALL']:
     targets = configs.keys()
 
   print 'Building for', ', '.join(targets)