TF-M: builds download source from external git repository

[JordanYates]

Describe the bug

Since the update to TFM v2.1.0, any builds of TF-M are cloning down sources from this repo: https://github.com/ARM-software/CMSIS_6

Offending commit: zephyrproject-rtos/trusted-firmware-m@3d54aca

To Reproduce

west build -b nrf9161dk/nrf9161/ns zephyr/samples/hello_world/ -p
Observe:

...
-- Build files have been written to: /home/jordan/code/zephyr/build/tfm/lib/ext/cmsis-subbuild
[1/9] Creating directories for 'cmsis-populate'
[1/9] Performing download step (git clone) for 'cmsis-populate'
Cloning into 'cmsis-src'...
remote: Enumerating objects: 28525, done.
remote: Counting objects: 100% (3662/3662), done.
remote: Compressing objects: 100% (977/977), done.
remote: Total 28525 (delta 3071), reused 3002 (delta 2624), pack-reused 24863
Receiving objects: 100% (28525/28525), 39.74 MiB | 5.76 MiB/s, done.
Resolving deltas: 100% (21001/21001), done.
HEAD is now at d0c460c1 CMSIS-Core(M): Add Cortex-M52 support : add core_m52.h
[2/9] Performing update step for 'cmsis-populate'
...

Expected behavior

Builds should not be pulling down sources from the internet.
Builds should complete regardless of internet connectivity state.

Impact

"Untrusted" (Not zephyrproject) source being pulled down from the internet on each build.
Unable to build TF-M applications without an internet connection.
40MB download for every TF-M build in CI.
Feels like it violates some sort of Zephyr guideline, or should.

Environment (please complete the following information):

~/code/zephyr/zephyr$ git describe
v3.7.0-rc3-73-gac52bd629d7

[tomi-font]

#76094 is a quick fix.
The proper way would be to fork the CMSIS v6 repository and add it as a new West project to Zephyr.

[fabiobaltieri]

cc @nashif @aescolar