AP_Param: don't enable param backup on all boards

this fixes an issue with resetting of parameters when going between
4.4.x and 4.5.x on MatekH743, and on any other board using flash
storage where the storage size has increased from 16k to 32k between
4.4.x and 4.5.x

The problem is that when you update to 4.5.x the parameter code stored
a backup of parameters in the StorageParamBak storage region which is
in the last section of storage. When you downgrade to 4.4.x the
AP_FlashStorage::load_sector() code tries to load this data and gets
an error as it is beyond the end of the available 16k storage. This
triggers an erase_all() and loss of parameters

libraries/AP_Param/AP_Param.cpp

@@ -50,6 +50,15 @@ uint16_t AP_Param::sentinal_offset;
 // singleton instance
 AP_Param *AP_Param::_singleton;
 
+#ifndef AP_PARAM_STORAGE_BAK_ENABLED
+// we only have a storage region for backup storage if we have at
+// least 32768 bytes or storage. We also don't enable when using flash
+// storage as this can lead to loss of storage when updating to a
+// larger storage size
+#define AP_PARAM_STORAGE_BAK_ENABLED (HAL_STORAGE_SIZE>=32768) && !defined(STORAGE_FLASH_PAGE)
+#endif
+
+
 #define ENABLE_DEBUG 0
 
 #if ENABLE_DEBUG
@@ -128,8 +137,10 @@ const AP_Param::param_defaults_struct AP_Param::param_defaults_data = {
 // storage object
 StorageAccess AP_Param::_storage(StorageManager::StorageParam);
 
+#if AP_PARAM_STORAGE_BAK_ENABLED
 // backup storage object
 StorageAccess AP_Param::_storage_bak(StorageManager::StorageParamBak);
+#endif
 
 // flags indicating frame type
 uint16_t AP_Param::_frame_type_flags;
@@ -138,7 +149,9 @@ uint16_t AP_Param::_frame_type_flags;
 void AP_Param::eeprom_write_check(const void *ptr, uint16_t ofs, uint8_t size)
 {
     _storage.write_block(ofs, ptr, size);
+#if AP_PARAM_STORAGE_BAK_ENABLED
     _storage_bak.write_block(ofs, ptr, size);
+#endif
 }
 
 bool AP_Param::_hide_disabled_groups = true;
@@ -337,14 +350,19 @@ void AP_Param::check_var_info(void)
 bool AP_Param::setup(void)
 {
     struct EEPROM_header hdr {};
-    struct EEPROM_header hdr2 {};
 
     // check the header
     _storage.read_block(&hdr, 0, sizeof(hdr));
+
+#if AP_PARAM_STORAGE_BAK_ENABLED
+    struct EEPROM_header hdr2 {};
     _storage_bak.read_block(&hdr2, 0, sizeof(hdr2));
+#endif
+
     if (hdr.magic[0] != k_EEPROM_magic0 ||
         hdr.magic[1] != k_EEPROM_magic1 ||
         hdr.revision != k_EEPROM_revision) {
+#if AP_PARAM_STORAGE_BAK_ENABLED
         if (hdr2.magic[0] == k_EEPROM_magic0 &&
             hdr2.magic[1] == k_EEPROM_magic1 &&
             hdr2.revision == k_EEPROM_revision &&
@@ -353,14 +371,17 @@ bool AP_Param::setup(void)
             INTERNAL_ERROR(AP_InternalError::error_t::params_restored);
             return true;
         }
+#endif // AP_PARAM_STORAGE_BAK_ENABLED
         // header doesn't match. We can't recover any variables. Wipe
         // the header and setup the sentinal directly after the header
         Debug("bad header in setup - erasing");
         erase_all();
     }
 
+#if AP_PARAM_STORAGE_BAK_ENABLED
     // ensure that backup is in sync with primary
     _storage_bak.copy_area(_storage);
+#endif
 
     return true;
 }