Check dir existence before finding files for OTATOOLS_DEPS.

This avoids building errors in branches that don't include such projects
(e.g. external/avb or system/update_engine). Note that although it
won't actually build otatools in such branches, it fails while parsing
the Makefile.

Bug: 112293933
Test: Rename external/avb. `m otatools` passes.
Change-Id: I334239fb23b1795ec77049ba9b81b288d2b4e2da