AP_FlashStorage: fixed build error with -O0

libraries/AP_FlashStorage/AP_FlashStorage.cpp

@@ -315,7 +315,9 @@ bool AP_FlashStorage::write_all()
     debug("write_all to sector %u at %u with reserved_space=%u\n",
            current_sector, write_offset, reserved_space);
     for (uint16_t ofs=0; ofs<storage_size; ofs += max_write) {
-        uint8_t n = MIN(max_write, storage_size-ofs);
+        // local variable needed to overcome problem with MIN() macro and -O0
+        const uint8_t max_write_local = max_write;
+        uint8_t n = MIN(max_write_local, storage_size-ofs);
         if (!all_zero(ofs, n)) {
             if (!write(ofs, n)) {
                 return false;