GitLab

* Nano ESP32: fix previous merge issues

* platform: remove previous build options if file is missing

"touch" would create the file if not present, but not delete its
contents if a previous run left the file in the build dir.

* platform: make debug_custom.json file customizable by board

* platform: fix default debug prefix

"debug.toolchain.prefix" must end with a dash, since only the tool name is
appended to this string.

The reason this is not a major issue is that the "debug_custom.json" file
(copied in the sketch directory when debugging is enabled) forces its own
prefix. And to make things more interesting, the "toolchainPrefix" entry
in that file should _not_ end with a dash.

* Nano ESP32: add debugging support