MLECO-3552: Workaround for toolchain preset

There is a currently known bug that ${fileDir} in included presets
evaluates to the path of parent file, not path of the file with
${fileDir} macro.
To be able to use the toolchain preset in case of MLEK included as
submodule - add an environment variable to be set by the parent project.
Since this variable is not set by the MLEK project - it does not affect
the functionality in MLEK.

Signed-off-by: Maksims Svecovs <maksims.svecovs@arm.com>
Change-Id: I4b789a12533f0474ed5490cf7a793e953a110898
diff --git a/scripts/cmake/toolchains/toolchain-preset.json b/scripts/cmake/toolchains/toolchain-preset.json
index 728547f..30b5366 100644
--- a/scripts/cmake/toolchains/toolchain-preset.json
+++ b/scripts/cmake/toolchains/toolchain-preset.json
@@ -13,7 +13,7 @@
             "cacheVariables": {
                 "CMAKE_TOOLCHAIN_FILE": {
                     "type": "STRING",
-                    "value": "${sourceDir}/scripts/cmake/toolchains/bare-metal-gcc.cmake"
+                    "value": "$env{MLEK_BASE_DIR}scripts/cmake/toolchains/bare-metal-gcc.cmake"
                 }
             }
         },
@@ -24,7 +24,7 @@
             "cacheVariables": {
                 "CMAKE_TOOLCHAIN_FILE": {
                     "type": "STRING",
-                    "value": "${sourceDir}/scripts/cmake/toolchains/bare-metal-armclang.cmake"
+                    "value": "$env{MLEK_BASE_DIR}scripts/cmake/toolchains/bare-metal-armclang.cmake"
                 }
             }
         },
@@ -35,7 +35,7 @@
             "cacheVariables": {
                 "CMAKE_TOOLCHAIN_FILE": {
                     "type": "STRING",
-                    "value": "${sourceDir}/scripts/cmake/toolchains/native-gcc.cmake"
+                    "value": "$env{MLEK_BASE_DIR}scripts/cmake/toolchains/native-gcc.cmake"
                 }
             }
         }