build/cmake: lock contrib directory

Prevents concurrent build of the same contrib
causing build failures.

Change-Id: I11a3664340e8a16c5c76926638a5f29f6dc0137c

CMakeLists.txt

@@ -126,6 +126,7 @@ if (BUILD_CONTRIB)
             set(ENV{LD} $ENV{TOOLCHAIN}/bin/ld)
         endif()
 
+        file(LOCK ${CONTRIB_BUILD_PATH} DIRECTORY)
         execute_process(
             COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/contrib/bootstrap --host=${TARGET}
             WORKING_DIRECTORY ${CONTRIB_BUILD_PATH})
@@ -133,6 +134,7 @@ if (BUILD_CONTRIB)
             WORKING_DIRECTORY ${CONTRIB_BUILD_PATH})
         execute_process(COMMAND make ${CONTRIB_BUILD_FLAGS}
             WORKING_DIRECTORY ${CONTRIB_BUILD_PATH})
+        file(LOCK ${CONTRIB_BUILD_PATH} DIRECTORY RELEASE)
     else()
         set(SCRIPTS_DIR ${CMAKE_CURRENT_SOURCE_DIR}/extras/scripts)
         execute_process(