App storage functionality use simplification

[iartemov-ledger]

Based on #56 and replaces it
Blocked by LedgerHQ/ledger-secure-sdk#865 or sooner a new PR targeting master
Pair review: LedgerHQ/app-boilerplate#147

Checklist

• No N_ variables anymore, only app_storage API
• defines accessor macros for READ and WRITE operations
• cached in RAM config part of storage data
• compile-time check for the data storage size
• backup and restore tested locally with app_storage_utils.py script (again)
• current tests fixes:
  • Stax
  • Flex
  • NanoX
  • NanoSP
  • NanoS ?
• New --app-storage tests (requires support in Speculos, maybe a special test app needed)

• App update process has been followed
• Target branch is develop
• Application version has been bumped