Commonise flash-based OTA. Add h7 support.

src/config.h

@@ -149,3 +149,11 @@
 #define MG_EPOLL_ADD(c)
 #define MG_EPOLL_MOD(c, wr)
 #endif
+
+#ifndef MG_ENABLE_STM32H5
+#define MG_ENABLE_STM32H5 0
+#endif
+
+#ifndef MG_ENABLE_STM32H7
+#define MG_ENABLE_STM32H7 0
+#endif