TDBStore: remove get_flash_bounds input constraint

Handle the case where the entirety of flash (size = 0) is required
for a flash memory starting at address 0, instead of erroring out.

Author: Kyle Kearney

features/storage/kvstore/tdbstore/TDBStore.cpp

@@ -1529,8 +1529,8 @@ int TDBStore::get_flash_bounds_from_config(bd_addr_t *start_address, bd_size_t *
 
     if (*start_address == 0) {
         if (*size == 0) {
-            flash.deinit();
-            return MBED_ERROR_INVALID_ARGUMENT;
+            //The block device will have all space from start address to the end of the flash
+            *size = flash.get_flash_size();
         }
 
         *start_address = flash_end_address - *size;

features/storage/kvstore/tdbstore/TDBStore.h

@@ -301,10 +301,9 @@ class TDBStore : public KVStore {
 
     /**
      * @brief Get the TDBStore flash bounds from the configured start address and size.
-     * Configured start address/size must not both be 0.
      *
-     * @param[inout] start_address    Configured TDBStore start address in flash.
-     * @param[inout] size             Configured TDBStore size.
+     * @param[inout] start_address    Configured TDBStore start address in flash. 
+     * @param[inout] size             Configured TDBStore size. If 0, the size will be from the start address to the end of flash
      *
      * @returns MBED_SUCCESS                        Success.
      *          MBED_ERROR_INVALID_ARGUMENT         One of the arguments is NULL or both the configured start address and size are 0.