Make the packaging of VBOOT_SIGNER tools conditional.

Otherwise it may fail the targets that don't have
external/vboot_reference in the manifest.

Bug: 35467608
Test: lunch aosp_dragon-userdebug; m otatools-package
Test: lunch aosp_bullhead-userdebug; m otatools-package
Change-Id: I35700a4d7d0723992badcfb4a03be85d07a4b9a9
(cherry picked from commit d2aed430)