contrib: Ease the configuration of a custom cache directory.

It was previously possible by overriding the TARBALLS variable on each
make call, but that's inconvenient.  This changes allows specifying it
once, at bootstrap time.  If the TARBALLS environment is defined, it
is also honored (and takes precedence over the value provided at
bootstrap time).

* contrib/src/ffmpeg/rules.mak (ffmpeg): Because the $(TARBALLS)
variable is now made absolute (for clarity in the build output), do
not use a relative path.
* contrib/src/onnx/rules.mak (onnx): Likewise.
* contrib/src/vpx/rules.mak (libvpx): Likewise.
* contrib/src/x264/rules.mak: Likewise.

Change-Id: I07497b095938c7885159f44753ead2814ed2a2fe