fix(cmakev2/idf): fix IDF_TOOLCHAIN build property for linux target

The IDF_TOOLCHAIN build property is currently incorrectly set to the
default `gcc` value for the linux target, whereas it should be empty.
This misconfiguration causes confusion for components like `soc`, which
adjust toolchain options based on the
IDF_TOOLCHAIN(CONFIG_IDF_TOOLCHAIN_GCC) build property's setting.

When sdkconfig is generated, the IDF_TOOLCHAIN build property is passed
as an environmental variable to kconfgen, and the CONFIG_IDF_TOOLCHAIN
configuration option is set based on this variable. Additionally, the
CONFIG_IDF_TOOLCHAIN_GCC and CONFIG_IDF_TOOLCHAIN_CLANG configuration
options are set accordingly. Subsequently, CONFIG_IDF_TOOLCHAIN_GCC is
used in several places, such as `components/soc/project_include.cmake`,
to configure the toolchain (compiler flags) by invoking functions from
`tools/cmake/toolchain_flags.cmake`, which is included only for
non-linux targets. As a result the configuration fails, because
functions from `tools/cmake/toolchain_flags.cmake` are not available on
linux target.

Since the IDF_TOOLCHAIN cmake cache variable is actually set in the
`tools/cmake/toolchain.cmake` file, the IDF_TOOLCHAIN build property
should be set after the toolchain is initialized in cmakev2's project
initialization. Note that each toolchain file, except for linux,
includes `toolchain.cmake`, which in turn includes
`toolchain_flags.cmake`. This means the IDF_TOOLCHAIN cmake cache
variable is set for every target except linux, because the toolchain
file for linux is empty. As a result CONFIG_IDF_TOOLCHAIN is empty and
CONFIG_IDF_TOOLCHAIN_GCC not set as for cmakev1.

Signed-off-by: Frantisek Hrbata <frantisek.hrbata@espressif.com>

tools/cmakev2/idf.cmake

@@ -266,16 +266,15 @@ endfunction()
 #[[
     __init_toolchain()
 
-    Determine the IDF_TOOLCHAIN value from the IDF_TOOLCHAIN environment
-    variable or the CMake cache variable. If none of these are set, use the
-    default gcc toolchain. Ensure there are no inconsistencies in the
-    IDF_TOOLCHAIN values set in different locations. Also ensure that the
+    Determine the toolchain file, set IDF_TOOLCHAIN_FILE build property and
+    global CMAKE_TOOLCHAIN_FILE CMake variable. Also ensure that the
     CMAKE_TOOLCHAIN_FILE is set to the correct file according to the current
     IDF_TARGET.
 
-    Set the IDF_TOOLCHAIN and IDF_TOOLCHAIN_FILE build properties. Also,
-    configure the IDF_TOOLCHAIN CMake cache variable and set the
-    CMAKE_TOOLCHAIN_FILE global variable.
+    Note: The IDF_TOOLCHAIN build property is set after the toolchain
+    configuration in ``idf_project_init``. The ``tools/cmake/toolchain.cmake``
+    is included in the toolchain file and it sets the IDF_TOOLCHAIN variable in
+    CMake's cache.
 #]]
 function(__init_toolchain)
     set(cache_toolchain $CACHE{IDF_TOOLCHAIN})
@@ -318,9 +317,7 @@ function(__init_toolchain)
         idf_die("Toolchain file ${toolchain_file} not found")
     endif()
 
-    set(IDF_TOOLCHAIN ${toolchain} CACHE STRING "IDF Build Toolchain Type")
     set(CMAKE_TOOLCHAIN_FILE "${toolchain_file}" PARENT_SCOPE)
-    idf_build_set_property(IDF_TOOLCHAIN "${toolchain}")
     idf_build_set_property(IDF_TOOLCHAIN_FILE "${toolchain_file}")
 endfunction()
 

tools/cmakev2/project.cmake

@@ -560,6 +560,11 @@ macro(idf_project_init)
         # Ensure this function is executed only once throughout the entire
         # project.
 
+        # The IDF_TOOLCHAIN variable is established as a CMake cache variable
+        # during the toolchain initialization process in
+        # ``tools/cmake/toolchain.cmake``.
+        idf_build_set_property(IDF_TOOLCHAIN "${IDF_TOOLCHAIN}")
+
         # Warn about the use of deprecated variables.
         deprecate_variable(COMPONENTS)
         deprecate_variable(EXCLUDE_COMPONENTS)