Add UF2TOOL_PATH CMake variable to avoid download during build time

By default, `uf2tool` source will be downloaded from `hex` mirrors. Setting
`UF2TOOL_PATH` variable allow to use a local checkout of the source tree, thus
removing this download at build time.

Signed-off-by: Paul Guyot <pguyot@kallisys.net>

Author: pguyot

doc/src/build-instructions.md

@@ -71,6 +71,14 @@ Source code is organized as follows:
 
 The `src` directory is broken up into the core platform-independent AtomVM library (`libAtomVM`), and platform-dependent code for each of the supported platforms  (Generic UNIX, ESP32, and STM32).
 
+## External dependencies
+
+### `uf2tool`
+
+AtomVM depends on `uf2tool`. It is used to pack both native and Erlang/Elixir/Gleam code for RP2. uf2tool is downloaded automatically by `rebar3` from `hex` mirrors.
+
+It is possible to use a local copy of uf2tool by setting `UF2TOOL_PATH` variable to a path to a source checkout of [`uf2tool`](https://github.com/pguyot/uf2tool) when invoking CMake.
+
 ## Platform Specific Build Instructions
 
 * [Generic UNIX](#building-for-generic-unix)

tools/uf2tool/CMakeLists.txt

@@ -21,6 +21,13 @@
 cmake_minimum_required (VERSION 3.13)
 project (UF2Tool)
 
+set(UF2TOOL_PATH "" CACHE PATH "Path to UF2Tool source tree. If unset, hex package will be used")
+
+if(NOT UF2TOOL_PATH STREQUAL "")
+    file(MAKE_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/_checkouts)
+    file(CREATE_LINK ${UF2TOOL_PATH} ${CMAKE_CURRENT_BINARY_DIR}/_checkouts/uf2tool SYMBOLIC)
+endif()
+
 file(COPY ${CMAKE_CURRENT_SOURCE_DIR}/rebar.config
     DESTINATION ${CMAKE_CURRENT_BINARY_DIR}
 )