- Remove vpxdec and vpxenc from the exclude list.
- vpx{dec,enc}.sh: Updates to support finding their executable when
  LIBVPX_BIN_PATH is setup for the examples.
- tools_common.sh: New library function, vpx_tool_path(). Provides
  support for finding the exectuables in vpx{dec,enc}.sh.

Change-Id: I730f11cceb44646491a7a7ff58603a4a760129ef