The previous commit bd5ceee484f201b90a384636ba12de86bd330cba removed
the definition of the global constants
- mbedtls_test_ca_crt_rsa_len,
- mbedtls_test_cli_crt_rsa_len,
- mbedtls_test_ca_crt_rsa, and
- mbedtls_test_cli_crt_rsa.
This commit restores these to maintain ABI compatibility.

Further, it was noticed that without SHA256_C being enabled the
previous code failed to compile because because the SHA1 resp. SHA256
certificates were only defined when the respective SHAXXX_C options
were set, but the emission of the global variable mbedtls_test_ca_crt
was unconditionally defined through the SHA256
certificate. Previously, the RSA SHA1 certificate was unconditionally
defined and used for that.

As a remedy, this commit makes sure some RSA certificate is defined
and exported through the following rule:
1. If SHA256_C is active, define an RSA SHA256 certificate and export
   it as mbedtls_test_ca_crt. Also, define SHA1 certificates only if
   SHA1_C is set.
2. If SHA256_C is not set, always define SHA1 certificate and export
   it as mbedtls_test_ca_crt.