RISC-V PVM: accept input

What

This MR uses the RISC-V PVM provide_input and provide_metadata functions in the dummy PVM, updating the node side of the PVM accordingly. In order to enable testing with a kernel in CI, in the absence of an installer kernel, the backend is now configured to initialise the state with the dummy kernel if the empty kernel is passed to install_boot_sector.

Closes RV-76.

Why

It is a step towards integrating the RISC-V PVM in the rollup node.

Manually testing the MR

The test_rollup_node_advances_pvm_state protocols ~kind ~internal:true test has been disabled because the dummy kernel doesn't parse transfers. The remaining ~internal:false variant passes.

make && dune exec ./tezt/tests/main.exe -- riscv