When used as a submodule, PSA source files pick up the wrong config.h

[gilles-peskine-arm]

Description

When Mbed Crypto is used as a submodule in Mbed TLS, only the config.h of Mbed TLS should be used. But PSA source files use the ones in the submodule.

Steps to reproduce:

git clone https://github.com/ARMmbed/mbedtls
cd mbedtls
git submodule update
rm crypto/include/mbedtls/config.h
make

Expected behavior: since Mbed Crypto is used as a submodule in Mbed TLS, the config.h file under crypto should not be used.

Actual behavior:

…
  CC    psa_crypto.c
In file included from ../include/psa/crypto.h:25:0,
                 from psa_crypto.c:31:
../include/psa/crypto_platform.h:41:31: fatal error: ../mbedtls/config.h: No such file or directory
compilation terminated.
Makefile:146: recipe for target 'psa_crypto.o' failed
make[2]: *** [psa_crypto.o] Error 1
Makefile:152: recipe for target 'libmbedcrypto.a' failed
make[1]: *** [libmbedcrypto.a] Error 2
Makefile:19: recipe for target 'lib' failed
make: *** [lib] Error 2

Validation: to make sure that the wrong config.h is not used, we should do a build where its contents are replaced by something that will cause a build error, e.g. #barf.

Issue request type

[ ] Question
[ ] Enhancement
[x] Bug

[Patater]

#include "../mbedtls/config.h" shouldn't be used. Use #include "mbedtls/config.h instead, so that the submodule-building makefiles can change which directory to use to get mbedtls include files from.