To add support for NVRAM save and load operations for Rust

[iartemov-ledger]

As it is don for C-apps in #552.
The linking is done rather differently for Rust:

1. LLD: https://github.com/LedgerHQ/ledger-device-rust-sdk/blob/master/ledger_secure_sdk_sys/link.ld#L77
2. relocation: https://github.com/LedgerHQ/ledger-device-rust-sdk/blob/master/ledger_secure_sdk_sys/link_wrap.sh
3. then at app startup these calls: https://github.com/LedgerHQ/ledger-device-rust-sdk/blob/13ede06b1c28063689aa487a08ff6a00a344fd18/ledger_secure_sdk_sys/src/c/src.c#L313 (namely link_pass_nvram() and link_pass_ram())

We may want to look for:

• _nvm_data_start - for the beginning of NVRAM zone
• _nvram_prev_run or _envram_data - for its end
  in a separate .nvm_data section (another segment).

There are sys_nvm_write() calls at the app-boilerplate-rust start-up at address c0df2e00 (=.rodata, _erelocs, _nvram_data - relocations are being made ?)

.nvm_data section is linked to flash2 segment, different form flash with .text. So we may pass by chance here https://github.com/LedgerHQ/speculos/blob/master/src/launcher.c#L247 aading an additional page to .text section. What if the writable NVRAM is bigger ?

[bboilot-ledger]

FYI #467