Dualcpu: Generate tfm_mailbox_config.h to build folder

tfm_mailbox_config.h is generated to source code folder. Git will keep
notifying developers to add this new file if developers build TF-M in
the source code folder.

Make tfm_mailbox_config.h to be generated in build folder instead.

Change-Id: I522228bef6329ae1ea1f3b918f54d8214d6f3664
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/cmake/install.cmake b/cmake/install.cmake
index 2af1476..fb92991 100644
--- a/cmake/install.cmake
+++ b/cmake/install.cmake
@@ -53,7 +53,7 @@
     install(FILES       ${INTERFACE_INC_DIR}/multi_core/tfm_multi_core_api.h
                         ${INTERFACE_INC_DIR}/multi_core/tfm_ns_mailbox.h
                         ${INTERFACE_INC_DIR}/multi_core/tfm_mailbox.h
-                        ${INTERFACE_INC_DIR}/multi_core/tfm_mailbox_config.h
+                        ${CMAKE_BINARY_DIR}/generated/interface/include/tfm_mailbox_config.h
             DESTINATION ${INSTALL_INTERFACE_INC_DIR})
 elseif (NOT TFM_PSA_API)
     install(FILES       ${CMAKE_BINARY_DIR}/generated/interface/include/tfm_veneers.h