refactor SHA-1 certificate defintions and assignment

As per refactoring suggestion that I made in #1520.
diff --git a/library/certs.c b/library/certs.c
index f857295..02c2215 100644
--- a/library/certs.c
+++ b/library/certs.c
@@ -176,7 +176,7 @@
 
 #endif /* MBEDTLS_SHA256_C */
 
-#if !defined(TEST_CA_CRT_RSA_SOME) || defined(MBEDTLS_SHA1_C)
+#if  defined(MBEDTLS_SHA1_C) || !defined(TEST_CA_CRT_RSA_SOME) || !defined(TEST_SRV_CRT_RSA_SOME)
 #define TEST_CA_CRT_RSA_SHA1                                            \
 "-----BEGIN CERTIFICATE-----\r\n"                                       \
 "MIIDhzCCAm+gAwIBAgIBADANBgkqhkiG9w0BAQUFADA7MQswCQYDVQQGEwJOTDER\r\n"  \
@@ -200,16 +200,6 @@
 "7Z2mCGDNMhjQc+BYcdnl0lPXjdDK6V0qCg1dVewhUBcW5gZKzV7e9+DpVA==\r\n"      \
 "-----END CERTIFICATE-----\r\n"
 
-#if !defined (TEST_CA_CRT_RSA_SOME)
-const char   mbedtls_test_ca_crt_rsa[]   = TEST_CA_CRT_RSA_SHA1;
-const size_t mbedtls_test_ca_crt_rsa_len = sizeof( mbedtls_test_ca_crt_rsa );
-#endif
-
-static const char mbedtls_test_ca_crt_rsa_sha1[] = TEST_CA_CRT_RSA_SHA1;
-
-#endif /* !TEST_CA_CRT_RSA_SOME || MBEDTLS_SHA1_C */
-
-#if !defined(TEST_SRV_CRT_RSA_SOME) || defined(MBEDTLS_SHA1_C)
 #define TEST_SRV_CRT_RSA_SHA1                                                  \
 "-----BEGIN CERTIFICATE-----\r\n"                                      \
 "MIIDNzCCAh+gAwIBAgIBAjANBgkqhkiG9w0BAQUFADA7MQswCQYDVQQGEwJOTDER\r\n" \
@@ -232,12 +222,19 @@
 "4mN4lW7gLdenN6g=\r\n"                                                 \
 "-----END CERTIFICATE-----\r\n";
 
+#if !defined (TEST_CA_CRT_RSA_SOME)
+const char   mbedtls_test_ca_crt_rsa[]   = TEST_CA_CRT_RSA_SHA1;
+const size_t mbedtls_test_ca_crt_rsa_len = sizeof( mbedtls_test_ca_crt_rsa );
+#endif
+
 #if !defined(TEST_SRV_CRT_RSA_SOME)
 const char mbedtls_test_srv_crt_rsa[] =  TEST_SRV_CRT_RSA_SHA1;
 const size_t mbedtls_test_srv_crt_rsa_len = sizeof( mbedtls_test_srv_crt_rsa );
 #endif
 
-#endif /* !TEST_SRV_CRT_RSA_SOME || MBEDTLS_SHA1_C */
+static const char mbedtls_test_ca_crt_rsa_sha1[] = TEST_CA_CRT_RSA_SHA1;
+
+#endif /* MBEDTLS_SHA1_C || !TEST_CA_CRT_RSA_SOME || !TEST_SRV_CRT_RSA_SOME  */
 
 const char mbedtls_test_ca_key_rsa[] =
 "-----BEGIN RSA PRIVATE KEY-----\r\n"