nanokvm-serial-console: build on Core, source NanoKVM OS

Sources core.func, tools.func, error_handler.func and api.func like the other
host tools, with load_functions, catch_errors, header_info (APP_TYPE=tools),
root_check, on_pve_host and the prompt_* helpers instead of its own colours,
messages and whiptail. The Source line points at NanoKVM OS, whose USB serial
function this is for. Arguments plan|status|apply|undo [--yes]; NANOKVM_SERIAL
for a NanoKVM that is not plugged in yet.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: dormancygrace