core_logic: simplify RPC mechanism

Instead of having a bunch of RPC status codes, simply use the RPC command
code to indicate that the coprocessor has received the command.
core_logic: Fix MCU nSS polarity bug
core_logic: Rename "cop_nss" to "cop_select"

It is not active low.
core_logic: rename "sys_miso" to "dev_miso"

That better indicates that it's the peripheral device MISO.
core_logic: Test device selects and FPGA config signals
core_logic: Tri-state FPGA config lines when not using them

The FPGA lines are also driven by the Serial Flash and FPGA, so avoid a
bus contention.
core_logic: Use MSB first mode

This seems to be the more standard mode.
core_logic: fix spurious cop_select setting in test