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>
2 files changed