Fix Mongoose and Frozen include paths

In preparation for making moving to lib.
Reduce amount of makefile copypasta a bit.

CL: none

PUBLISHED_FROM=70a016491f9605e37708385bdd698f48a8d64119

Author: Deomid Ryabkov

src/mgos_pppos.c

@@ -14,7 +14,7 @@
 #include "common/mg_str.h"
 #include "common/queue.h"
 
-#include "mongoose/mongoose.h"
+#include "mongoose.h"
 
 #include "mgos_net_hal.h"
 #include "mgos_sys_config.h"