Add protocol-explicit upload.tool properties required for pluggable discovery compatibility (#933)

A new flexible and powerful "pluggable discovery" system was added to the Arduino boards platform framework. This system
makes it easy for Arduino boards platform authors to use any arbitrary communication channel between the board and
development tools.

Boards platform configurations that use the old property syntax are automatically translated to the new syntax by
Arduino CLI:

https://arduino.github.io/arduino-cli/latest/platform-specification/#sketch-upload-configuration

> For backward compatibility with IDE 1.8.15 and older the previous syntax is still supported

This translation is only done in platforms that use the old syntax exclusively. If `pluggable_discovery` properties are
defined for the platform then the new pluggable discovery-style `upload.tool.<protocol_name>` properties must be defined
for each board as well.

This platform includes a "UF2 Devices" discovery tool for `uf2conv` protocol ports. This tool now uses the modern
Arduino pluggable discovery system. `pluggable_discovery` properties were added to `platform.txt` in order to configure
the Arduino development tools to use this pluggable discovery tool. The `upload.tool.<protocol_name>` properties in
`boards.txt` were not updated at that time.

Global properties of that name were added to platform.txt, but this approach is not provided for by the Arduino Platform
specification:

https://arduino.github.io/arduino-cli/latest/platform-specification/#sketch-upload-configuration

> A specific upload.tool.<protocol_name> property should be defined for every board in boards.txt:

Those missing properties caused uploads to fail for users of the recent versions of Arduino IDE and Arduino CLI with an
error of the form:

Error during Upload: Property 'upload.tool.<protocol_name>' is undefined

(where `<protocol_name>` is the protocol of the selected port, if any)

It is also important to provide compatibility with versions of Arduino development tools from before the introduction of
the modern pluggable discovery system. For this reason, the old style `<board ID>.upload.tool` properties are retained.
Old versions of the development tools will treat the `<board ID>.upload.tool.<protocol_name>` properties as an unused
arbitrary user defined property with no special significance and the new versions of the development tools will do the
same for the `upload.tool` properties.

Author: per1234