change logic to be clearer

cmake/stm32/common.cmake

@@ -10,11 +10,11 @@ foreach(FAMILY ${STM32_SUPPORTED_FAMILIES_LONG_NAME})
 endforeach()
 list(REMOVE_DUPLICATES STM32_SUPPORTED_FAMILIES_SHORT_NAME)
 
-if(NOT STM32_TOOLCHAIN_PATH AND NOT CMAKE_C_COMPILER)
-    set(STM32_TOOLCHAIN_PATH "/usr")
-    message(STATUS "No STM32_TOOLCHAIN_PATH specified, using default: " ${STM32_TOOLCHAIN_PATH})
-else()
-    if(NOT STM32_TOOLCHAIN_PATH)
+if(NOT STM32_TOOLCHAIN_PATH)
+    if(NOT CMAKE_C_COMPILER)
+        set(STM32_TOOLCHAIN_PATH "/usr")
+        message(STATUS "No STM32_TOOLCHAIN_PATH specified, using default: " ${STM32_TOOLCHAIN_PATH})
+    else()
         # keep only directory of compiler
         get_filename_component(STM32_TOOLCHAIN_PATH ${CMAKE_C_COMPILER} DIRECTORY)
         # remove the last /bin directory