cros_generate_update_payload: Import installed chromeos-common.sh.

Don't import chromeos-common.sh from platform2/installer, since it
might not be present in a minilayout.

Also stopped trying to import from /usr/lib/installer, since we never
install the script there in any environment I can find.

BUG=brillo:396
TEST=Built, build_image, network update a dut.

Change-Id: Ib00ea30bfb2c698edd03b502550691c52cda0557
Reviewed-on: https://chromium-review.googlesource.com/254462
Reviewed-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Commit-Queue: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
diff --git a/host/cros_generate_update_payload b/host/cros_generate_update_payload
index 186c496..896ba1d 100755
--- a/host/cros_generate_update_payload
+++ b/host/cros_generate_update_payload
@@ -31,12 +31,11 @@
 find_common_sh
 . "${SCRIPT_ROOT}/common.sh" || exit 1
 
-# Load functions and constants for chromeos-install
-# NOTE: Needs to be called from outside the chroot.
-. "/usr/lib/installer/chromeos-common.sh" &> /dev/null || \
+# Load functions and constants for chromeos-common.
 . "${SRC_ROOT}/platform2/installer/share/chromeos-common.sh" &> /dev/null || \
+. "/usr/share/misc/chromeos-common.sh" &> /dev/null || \
 . "./chromeos-common.sh" || \
-  die "Unable to load /usr/lib/installer/chromeos-common.sh"
+  die "Unable to load chromeos-common.sh"
 
 DEFINE_string image "" "The image that should be sent to clients."
 DEFINE_string src_image "" "Optional: a source image. If specified, this makes\