Change to positive checks

1. Checge to check for `MBEDTLS_ECP_RESTARTABLE` for all definitions
of `rs_ctx`.
2. Remove checks for `_ALT` when using `rs_ctx` as they cannot coexist
with the Restartable configuration.
diff --git a/library/ecdh.c b/library/ecdh.c
index 307ee1f..5fb06cc 100644
--- a/library/ecdh.c
+++ b/library/ecdh.c
@@ -212,7 +212,7 @@
     if( ( ret = ecdh_gen_public_restartable( &ctx->grp, &ctx->d, &ctx->Q,
                                              f_rng, p_rng, rs_ctx ) ) != 0 )
         return( ret );
-#endif /* MBEDTLS_ECDH_GEN_PUBLIC_ALT */
+#endif /* !MBEDTLS_ECDH_GEN_PUBLIC_ALT */
 
     if( ( ret = mbedtls_ecp_tls_write_group( &ctx->grp, &grp_len, buf, blen ) )
                 != 0 )
@@ -286,14 +286,14 @@
                       void *p_rng )
 {
     int ret;
-#if !defined(MBEDTLS_ECDH_GEN_PUBLIC_ALT)
+#if defined(MBEDTLS_ECP_RESTARTABLE)
     mbedtls_ecp_restart_ctx *rs_ctx = NULL;
 #endif
 
     if( ctx == NULL || ctx->grp.pbits == 0 )
         return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );
 
-#if defined(MBEDTLS_ECP_RESTARTABLE) && !defined(MBEDTLS_ECDH_GEN_PUBLIC_ALT)
+#if defined(MBEDTLS_ECP_RESTARTABLE)
     if( ctx->restart_enabled )
         rs_ctx = &ctx->rs;
 #endif
@@ -342,14 +342,14 @@
                       void *p_rng )
 {
     int ret;
-#if !defined(MBEDTLS_ECDH_COMPUTE_SHARED_ALT)
+#if defined(MBEDTLS_ECP_RESTARTABLE)
     mbedtls_ecp_restart_ctx *rs_ctx = NULL;
 #endif
 
     if( ctx == NULL || ctx->grp.pbits == 0 )
         return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );
 
-#if defined(MBEDTLS_ECP_RESTARTABLE) && !defined(MBEDTLS_ECDH_GEN_PUBLIC_ALT)
+#if defined(MBEDTLS_ECP_RESTARTABLE)
     if( ctx->restart_enabled )
         rs_ctx = &ctx->rs;
 #endif