[nrf toup] cmake: only unset CMAKE_C_COMPILER for first image

The C compiler does not change between the different images.
Add a check to ensure that the C compiler variable is only
unset for the first boilerplate execution.

Signed-off-by: Håkon Øye Amundsen <[email protected]>
(cherry picked from commit 33c0613)
(cherry picked from commit 6c85501)
(cherry picked from commit 05270ea)
Signed-off-by: Martí Bolívar <[email protected]>

Author: hakonfam

cmake/target_toolchain.cmake

@@ -38,9 +38,12 @@ endif()
 
 # The 'generic' compiler and the 'target' compiler might be different,
 # so we unset the 'generic' one and thereby force the 'target' to
-# re-set it.
-unset(CMAKE_C_COMPILER)
-unset(CMAKE_C_COMPILER CACHE)
+# re-set it. This is only needed for the first boilerplate execution as
+# the remaining boilerplate executions will use the same C compiler.
+if (FIRST_BOILERPLATE_EXECUTION)
+  unset(CMAKE_C_COMPILER)
+  unset(CMAKE_C_COMPILER CACHE)
+endif()
 
 # A toolchain consist of a compiler and a linker.
 # In Zephyr, toolchains require a port under cmake/toolchain/.