Makefile: Pack brillo_update_payload and shflags into otatools.

brillo_update_payload and shflags are the scripts to generate A/B OTA
package (payload). Include them into otatools.zip.

brillo_update_payload was already included in previous CL. Move it to
the folder for executables.

Bug: 25715402
Change-Id: Idf9c05767c24c55d058cc30e696f06d4eb5fd53b
diff --git a/core/Makefile b/core/Makefile
index 3c1c84e..ee15f12 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1425,11 +1425,10 @@
   $(HOST_OUT_EXECUTABLES)/img2simg \
   $(HOST_OUT_EXECUTABLES)/boot_signer \
   $(HOST_OUT_EXECUTABLES)/fec \
+  $(HOST_OUT_EXECUTABLES)/brillo_update_payload \
+  $(HOST_OUT_EXECUTABLES)/lib/shflags/shflags \
   $(HOST_OUT_EXECUTABLES)/delta_generator
 
-OTATOOLS += \
-  system/update_engine/scripts/brillo_update_payload
-
 # Shared libraries.
 OTATOOLS += \
   $(HOST_LIBRARY_PATH)/libc++$(HOST_SHLIB_SUFFIX) \