Instructions for submodule initialization do not recurse submodules

[danielbernard]

Summary

Current instructions assume that only the top-level repository has submodules.
Since the submodule tf-psa-crypto includes framework as a submodule, submodules need to be initialized/updated recursively.

System information

Mbed TLS version (number or commit id): 67ba14c
Operating system and version: N/A
Configuration (if not default, please attach mbedtls_config.h): N/A
Compiler and options (if you used a pre-built binary, please indicate how you obtained it): N/A
Additional environment information: N/A

Expected behavior

After initializing submodules from the top-level, make rules should compile the build.

Actual behavior

Nested submodules are not initialized with the current instructions, and warnings from the top-level build don't catch that the sub-submodule is missing.

$ make
make[1]: Entering directory '/home/code/mbedtls/library'
  Gen   ../tf-psa-crypto/core/psa_crypto_driver_wrappers.h ../tf-psa-crypto/core/psa_crypto_driver_wrappers_no_static.c
Traceback (most recent call last):
  File "/home/code/mbedtls/library/../tf-psa-crypto/scripts/generate_driver_wrappers.py", line 22, in <module>
    from mbedtls_framework import build_tree
ModuleNotFoundError: No module named 'mbedtls_framework'
make[1]: *** [Makefile:397: ../tf-psa-crypto/core/psa_crypto_driver_wrappers.h] Error 1
make[1]: Leaving directory '/home/code/mbedtls/library'
make: *** [Makefile:41: lib] Error 2

Steps to reproduce

Perform a clean clone without submodule initialization and per the guide perform a non-recursive submodule update/initialization.

Additional information

Propose updating instructions and warnings to reference recursive submodule initialization:
git submodule update --init --recursive