wifi: hostap: Fixing naming collisions for some TLS configurations

-Mbed TLS has come up with its own naming-convention with regards
 to certain Mbed TLS configurations for TLS/DTLS and X.509. This
 commit fixes a couple of them by depending on MBEDTLS_BUILTIN
 which is set when Kconfig.tls-generic is used
 1. Make MBEDTLS_PEM_CERTIFICATE_FORMAT depend on MBEDTLS_BUILTIN
    The proper name for this functionaity is MBEDTL_PEM_PARSE_C and
    MBEDTLS_PEM_WRITE_C
 2. Make MBEDTLS_SERVER_NAME_INDICATION depend on MBEDTLS_BUILTIN
    The proper name for this is MBEDTLS_SSL_SERVER_NAME_INDICATION

Signed-off-by: Frank Audun Kvamtrø <frank.kvamtro@nordicsemi.no>