Remove _variant_bd_erase_unit_size

In some cases, it is possible that every erase unit in area 0
has the same size, but they are still different than in area 1.
Remove the flag for varying erase sizes and instead check from
flash, what is the erase size of the current unit.

Author: Jarno Lamsa

features/storage/kvstore/tdbstore/TDBStore.cpp

@@ -126,7 +126,7 @@ static uint32_t calc_crc(uint32_t init_crc, uint32_t data_size, const void *data
 TDBStore::TDBStore(BlockDevice *bd) : _ram_table(0), _max_keys(0),
     _num_keys(0), _bd(bd), _buff_bd(0),  _free_space_offset(0), _master_record_offset(0),
     _master_record_size(0), _is_initialized(false), _active_area(0), _active_area_version(0), _size(0),
-    _area_params{}, _prog_size(0), _work_buf(0), _key_buf(0), _variant_bd_erase_unit_size(false), _inc_set_handle(0)
+    _area_params{}, _prog_size(0), _work_buf(0), _key_buf(0), _inc_set_handle(0)
 {
     for (int i = 0; i < _num_areas; i++) {
         _area_params[i] = { 0 };
@@ -186,19 +186,19 @@ void TDBStore::calc_area_params()
 
     memset(_area_params, 0, sizeof(_area_params));
     size_t area_0_size = 0;
-    bd_size_t prev_erase_unit_size = _bd->get_erase_size(area_0_size);
-    _variant_bd_erase_unit_size = 0;
 
     while (area_0_size < bd_size / 2) {
         bd_size_t erase_unit_size = _bd->get_erase_size(area_0_size);
-        _variant_bd_erase_unit_size |= (erase_unit_size != prev_erase_unit_size);
         area_0_size += erase_unit_size;
     }
 
     _area_params[0].address = 0;
     _area_params[0].size = area_0_size;
     _area_params[1].address = area_0_size;
     _area_params[1].size = bd_size - area_0_size;
+
+    // The areas must be of same size
+    MBED_ASSERT(_area_params[0].size == _area_params[1].size);
 }
 
 
@@ -1487,14 +1487,8 @@ void TDBStore::offset_in_erase_unit(uint8_t area, uint32_t offset,
                                     uint32_t &offset_from_start, uint32_t &dist_to_end)
 {
     uint32_t bd_offset = _area_params[area].address + offset;
-    if (!_variant_bd_erase_unit_size) {
-        uint32_t eu_size = _buff_bd->get_erase_size();
-        offset_from_start = bd_offset % eu_size;
-        dist_to_end = eu_size - offset_from_start;
-        return;
-    }
-
     uint32_t agg_offset = 0;
+
     while (bd_offset >= agg_offset + _buff_bd->get_erase_size(agg_offset)) {
         agg_offset += _buff_bd->get_erase_size(agg_offset);
     }

features/storage/kvstore/tdbstore/TDBStore.h

@@ -308,7 +308,6 @@ class TDBStore : public KVStore {
     uint32_t _prog_size;
     uint8_t *_work_buf;
     char *_key_buf;
-    bool _variant_bd_erase_unit_size;
     void *_inc_set_handle;
     void *_iterator_table[_max_open_iterators];