Bugfix: won't rely on erase value to detect is a sector erased

When flashing a binary STLink won't skip writing padding which happens
to be the same value as flash's erase value. STM32L4 based targets
have an additional 8-bit of embedded ECC for each 64-bit word of data.
The initial value, when a sector is erased, for the ECC bits is 0xFF.
When you write the erase value to a given address these bits gets
modified to something different due to the ECC algoritm in use. The
visible bits are intact but difference in ECC value prevents flipping
any 1's to 0's. Only way to proceed is to erase the whole sector.

Author: Jammu Kekkonen

features/storage/kvstore/tdbstore/TDBStore.cpp

@@ -167,11 +167,7 @@ int TDBStore::erase_erase_unit(uint8_t area, uint32_t offset)
     uint32_t bd_offset = _area_params[area].address + offset;
     uint32_t eu_size = _buff_bd->get_erase_size(bd_offset);
 
-    int os_ret = _buff_bd->erase(bd_offset, eu_size);
-    if (os_ret) {
-        return MBED_ERROR_WRITE_FAILED;
-    }
-    return MBED_SUCCESS;
+    return _buff_bd->erase(bd_offset, eu_size);
 }
 
 void TDBStore::calc_area_params()
@@ -1498,29 +1494,19 @@ int TDBStore::is_erase_unit_erased(uint8_t area, uint32_t offset, bool &erased)
 int TDBStore::check_erase_before_write(uint8_t area, uint32_t offset, uint32_t size, bool force_check)
 {
     // In order to save init time, we don't check that the entire area is erased.
-    // Instead, whenever reaching an erase unit start, check that it's erased, and if not -
-    // erase it.
+    // Instead, whenever reaching an erase unit start erase it.
 
     while (size) {
         uint32_t dist, offset_from_start;
         int ret;
         offset_in_erase_unit(area, offset, offset_from_start, dist);
         uint32_t chunk = std::min(size, dist);
 
-        if (!offset_from_start || force_check) {
-            // We're at the start of an erase unit. Here (and only here, if not forced),
-            // check if it's erased.
-            bool erased;
-            ret = is_erase_unit_erased(area, offset, erased);
-            if (ret) {
+        if (offset_from_start == 0 || force_check) {
+            ret = erase_erase_unit(area, offset - offset_from_start);
+            if (ret != MBED_SUCCESS) {
                 return MBED_ERROR_WRITE_FAILED;
             }
-            if (!erased) {
-                ret = erase_erase_unit(area, offset - offset_from_start);
-                if (ret) {
-                    return MBED_ERROR_WRITE_FAILED;
-                }
-            }
         }
         offset += chunk;
         size -= chunk;