Improve AES hardware-only check

[daverodgman]

Description

Looking at #7384 (adds MBEDTLS_AES_USE_HARDWARE_ONLY), we complain with a #error message if this is set on x86 / x86_64 and MBEDTLS_AESNI_C is not set, and similar for Arm.

But if we set AES_USE_HARDWARE_ONLY on some other platform, there's no clean error about "not all prerequisites set", only an unexplained compile fail. This PR fixes that gap.

PR checklist

Please tick as appropriate and edit the reasons (e.g.: "backport: not needed because this is a new feature")

• changelog not required - minor enhancement, no change to functionality
• backport not required - not in 2.28
• tests not required - covered by existing

[tom-cosgrove-arm]

LGTM, I prefer the way this reverses things

Needs work

[gilles-peskine-arm]

LGTM

[tom-cosgrove-arm]

LGTM

[tom-cosgrove-arm]

And in case people hit the compiler warnings and search, the output you get when setting MBEDTLS_AES_USE_HARDWARE_ONLY on a platform that doesn't support it is

aes.c:41:2: error: "MBEDTLS_AES_USE_HARDWARE_ONLY defined, but not all prerequisites"
#error "MBEDTLS_AES_USE_HARDWARE_ONLY defined, but not all prerequisites"
 ^
  CC    bignum_mod_raw.c
aes.c:619:15: warning: variable 'RK' set but not used [-Wunused-but-set-variable]
    uint32_t *RK;
              ^
aes.c:616:75: warning: unused parameter 'key' [-Wunused-parameter]
int mbedtls_aes_setkey_enc(mbedtls_aes_context *ctx, const unsigned char *key,
                                                                          ^
  CC    camellia.c
aes.c:1066:48: warning: unused parameter 'ctx' [-Wunused-parameter]
int mbedtls_aes_crypt_ecb(mbedtls_aes_context *ctx,
                                               ^
aes.c:1068:47: warning: unused parameter 'input' [-Wunused-parameter]
                          const unsigned char input[16],
                                              ^
aes.c:1069:41: warning: unused parameter 'output' [-Wunused-parameter]
                          unsigned char output[16])
                                        ^

[daverodgman]

And in case people hit the compiler warnings and search, the output you get when setting MBEDTLS_AES_USE_HARDWARE_ONLY on a platform that doesn't support it is

aes.c:41:2: error: "MBEDTLS_AES_USE_HARDWARE_ONLY defined, but not all prerequisites"
#error "MBEDTLS_AES_USE_HARDWARE_ONLY defined, but not all prerequisites"
 ^
  CC    bignum_mod_raw.c
aes.c:619:15: warning: variable 'RK' set but not used [-Wunused-but-set-variable]
    uint32_t *RK;
              ^
aes.c:616:75: warning: unused parameter 'key' [-Wunused-parameter]
int mbedtls_aes_setkey_enc(mbedtls_aes_context *ctx, const unsigned char *key,
                                                                          ^
  CC    camellia.c
aes.c:1066:48: warning: unused parameter 'ctx' [-Wunused-parameter]
int mbedtls_aes_crypt_ecb(mbedtls_aes_context *ctx,
                                               ^
aes.c:1068:47: warning: unused parameter 'input' [-Wunused-parameter]
                          const unsigned char input[16],
                                              ^
aes.c:1069:41: warning: unused parameter 'output' [-Wunused-parameter]
                          unsigned char output[16])
                                        ^

It's annoying that the compiler generates all the other stuff instead of stopping at #error, but it's not really worth the code churn to fix it

[daverodgman]

Sorry, accidentally updated this by pushing to the wrong branch. I've reset to the point of review approval.

[tom-cosgrove-arm]

LGTM

[tom-cosgrove-arm]

LGTM