build_packages: fix inverted cli flag

shflags provides positive/negative variants for all boolean flags
for us automatically.  Declaring the flag as negative up front will
break this as we end up with --nonoworkon.

BUG=None
TEST=CQ passes

Change-Id: Ia638470ff2b1612aefac0bc0624bf3276459afe3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/1913490
Reviewed-by: Craig Hesling <hesling@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Nicolas Boichat <drinkcat@chromium.org>
diff --git a/build_packages b/build_packages
index 2bd3eff..51773d4 100755
--- a/build_packages
+++ b/build_packages
@@ -18,8 +18,8 @@
   "Use binary packages to bootstrap when possible."
 DEFINE_boolean usepkgonly "${FLAGS_FALSE}" \
   "Only use binary packages to bootstrap; abort if any are missing."
-DEFINE_boolean noworkon "${FLAGS_FALSE}" \
-  "Don't force-build workon packages."
+DEFINE_boolean workon "${FLAGS_TRUE}" \
+  "Force-build workon packages."
 DEFINE_boolean showoutput "${FLAGS_FALSE}" \
   "Show all output from parallel_emerge."
 DEFINE_boolean withautotest "${FLAGS_TRUE}" \
@@ -275,7 +275,7 @@
 
 # Build cros_workon packages when they are changed.
 CROS_WORKON_PKGS=()
-if [ "${FLAGS_noworkon}" -eq "${FLAGS_FALSE}" ]; then
+if [ "${FLAGS_workon}" -eq "${FLAGS_TRUE}" ]; then
   LIST_MODIFIED_PACKAGES="${CHROMITE_BIN}/cros_list_modified_packages"
   MODIFIED_PACKAGES=( $("${LIST_MODIFIED_PACKAGES}" --board=${FLAGS_board}) )
   info "cros_workon modified packages '${MODIFIED_PACKAGES[*]}' detected"