Commonise flash-based OTA. Add h7 support.

src/sys.h

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+// Copyright (c) 2023 Cesanta Software Limited
+// All rights reserved
+
+#pragma once
+
+// Flash information
+void *mg_flash_start(void);         // Return flash start address
+size_t mg_flash_size(void);         // Return flash size
+size_t mg_flash_sector_size(void);  // Return flash sector size
+size_t mg_flash_write_align(void);  // Return flash write align, minimum 4
+int mg_flash_bank(void);            // 0: not dual bank, 1: bank1, 2: bank2
+
+// Write, erase, swap bank
+bool mg_flash_write(void *addr, const void *buf, size_t len);
+bool mg_flash_erase(void *addr);  // Must be at sector boundary
+bool mg_flash_swap_bank(void);
+
+// Convenience functions to store data on a flash sector with wear levelling
+// If `sector` is NULL, then the last sector of flash is used
+bool mg_flash_load(void *sector, uint32_t key, void *buf, size_t len);
+bool mg_flash_save(void *sector, uint32_t key, const void *buf, size_t len);