The should pick other repositories from ../ only if both use and them
are submodules of ../ .

Only checking that ../zio is a directory is not enough, because it
can be a separate clone (old version, for example) or worse an
intermediate directory that doesn't host headers.

A user reported painful build errors for this reason.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>

- added gateware_install target, which automatically downloads and installs the FPGA gatewares
- improved submodule support. Now it looks for dependent submodules in the user-supplied path,
in the parent directory (a "super-module" repo) and if still not found, checks out local submodule copy.