Fix Twinkie overlay build

We just need the EC binary, disable the chromeos-bootimage ebuild which
is tricky to get right for this dummy configuration.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>

BUG=none
TEST=build_packages --board=twinkie

Change-Id: Iaf06e2150b245f9aa2bbf92261b4c9b44c4a059b
Reviewed-on: https://chromium-review.googlesource.com/676847
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>
Commit-Queue: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>
diff --git a/overlay-twinkie/make.conf b/overlay-twinkie/make.conf
index 2026804..99ed942 100644
--- a/overlay-twinkie/make.conf
+++ b/overlay-twinkie/make.conf
@@ -6,7 +6,6 @@
 USE="${USE} chromeless_tty"
 
 # Firwmare USE flags
-USE="${USE} bootimage"
 USE="${USE} cros_ec"
 
 # Build the standalone Twinkie EC firmware