Revert "Do not implicitely manage the installation of OPAM packages with the mirage tool"

We need the dependencies installed to calculate the linker options now.

This reverts commit c5355482a2f4efe473e0eb09fae73be7a8cb4432.
Don't tell the user to run "make depend"

We now do it automatically again.
Don't redirect "opam install" output to log

Installation can take a long time and the user will want to see what is
happening.
Discover and link C stub dependencies in --xen mode