Make config check include for configs examples more consistent

This way all config examples work when used like described in the README.
diff --git a/configs/config-picocoin.h b/configs/config-picocoin.h
index 4269872..26b24a9 100644
--- a/configs/config-picocoin.h
+++ b/configs/config-picocoin.h
@@ -66,6 +66,6 @@
 #define MBEDTLS_SHA1_C
 #define MBEDTLS_SHA256_C
 
-#include "check_config.h"
+#include "mbedtls/check_config.h"
 
 #endif /* MBEDTLS_CONFIG_H */