[posix] add posix config file configuration (#8612)

The ot-br-posix uses the posix platform's config file as its config
file. So the ot-br-posix can't set different configurations with posix
platform via the config file. This commit adds a config macro
`OPENTHREAD_POSIX_CONFIG_FILE` for ot-br-posix to specify a config
file to overwrite the posix platform's configurations.

src/posix/platform/openthread-posix-config.h

@@ -31,6 +31,10 @@
 
 #include "openthread-core-config.h"
 
+#ifdef OPENTHREAD_POSIX_CONFIG_FILE
+#include OPENTHREAD_POSIX_CONFIG_FILE
+#endif
+
 /**
  * @file
  * @brief