[TF-M] Move MCUBoot configs to bl2 specific config file

Hi all,

We decide to move MCUBoot specific configurations to TF-M/bl2/ext/mcuboot folder. This change is to decouple MCUBoot and TF-M configurations and make default_config.cmake clearer. I have proposed the patch set:

* [TF-M patchhttps://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/10560]

I'm grateful to receive any suggestion or enhancement from you.

Best Regards Jianliang Shen