Add flag to disable call to 'eclean'.

When triaging errors via bisecting good/bad packages, we need
build_image to NOT call eclean, or it messes up our bisecting
process.  Previously we've been manually editing the build_image
script to disable the call to eclean.  This CL adds a flag to do
this instead.

BUG=chromium:455306
TEST=Tested building images with & without flag.  Both cases worked
correctly.

Change-Id: I05302b06635f8be516e84d5e27c54ad947dafbe3
Reviewed-on: https://chromium-review.googlesource.com/344570
Commit-Ready: Caroline Tice <cmtice@chromium.org>
Tested-by: Caroline Tice <cmtice@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/build_image b/build_image
index 1c28a45..ebe4ac9 100755
--- a/build_image
+++ b/build_image
@@ -80,6 +80,8 @@
   "Toolset that this script is being used with."
 DEFINE_string version "" \
   "Overrides version number in name to this version."
+DEFINE_boolean eclean ${FLAGS_TRUE} \
+  "Do NOT call eclean before building the image (default is to call eclean)."
 
 # Parse command line.
 FLAGS "$@" || exit 1
@@ -158,7 +160,9 @@
 
 DEVKEYSDIR="/usr/share/vboot/devkeys"
 
-"${BOARD_ROOT}/build/bin/eclean" -d packages
+if [[ ${FLAGS_eclean} -eq ${FLAGS_TRUE} ]]; then
+  "${BOARD_ROOT}/build/bin/eclean" -d packages
+fi
 
 if [[ ${skip_blacklist_check} -ne ${FLAGS_FALSE} ]]; then
   check_blacklist