Introduce keygen --no-overwrite to avoid prompt

[gojimmypi]

Adds a new --no-overwrite option to the tools/keytools/keygen app.

This can be handy during CMake builds, for instance like this:

    add_custom_command(
        OUTPUT ${KEYSTORE} ${WOLFBOOT_SIGNING_PRIVATE_KEY}
        COMMAND "${KEYGEN_TOOL}" ${KEYTOOL_OPTIONS}
                --no-overwrite
                -g ${WOLFBOOT_SIGNING_PRIVATE_KEY}
                -keystoreDir ${CMAKE_CURRENT_BINARY_DIR}
        DEPENDS ${KEYGEN_TOOL}
        WORKING_DIRECTORY ${WOLFBOOT_ROOT}
        COMMENT "Generating keystore.c and signing private key"
        VERBATIM
    )

Otherwise, at least on Windows, the resulting spawned app will quietly await a response as a hidden process, typically keeping files locked, unable to build again, forcing a reboot if task manager won't terminate the process.

Reminder the regular if(EXIST ... is a configure-time check, not build-time. See docs.

Although a custom command could be used in this cmake example, it will be handy for other automation processes outside of cmake to have the "yes, I know the keystore might already exist, so don't prompt, and don't overwrite it" option.