Revert "Move board-specific setup to common.sh & add a kernel signing function"

This reverts commit 4474bf2d62fc90eeb5689d2eb3445c29c30d0042

This change broke the factory build for parrot canary. crbug.com/260805

Change-Id: I5b971fa597b5b0ba04e65ec6563d44f3ca80ac02
Reviewed-on: https://gerrit.chromium.org/gerrit/62244
Tested-by: Paul Taysom <taysom@chromium.org>
Reviewed-by: Albert Chaulk <achaulk@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Don Garrett <dgarrett@chromium.org>
diff --git a/build_image b/build_image
index 135f113..6eb0768 100755
--- a/build_image
+++ b/build_image
@@ -106,7 +106,12 @@
 
 parse_build_image_args
 
-board_specific_setup "${FLAGS_board}"
+for overlay in $(cros_list_overlays --board "$BOARD"); do
+  setup_sh="${overlay}/scripts/board_specific_setup.sh"
+  if [[ -e ${setup_sh} ]]; then
+    source "${setup_sh}"
+  fi
+done
 
 # TODO: <prebuild hook>