Preps for 2.1.0-rc1 release

[travis]: https://travis-ci.org/mcu-tools/mcuboot

[license]: https://github.com/mcu-tools/mcuboot/blob/main/LICENSE

This is MCUboot version 2.1.0-dev

This is MCUboot version 2.1.0-rc1

MCUboot is a secure bootloader for 32-bits microcontrollers. It defines a

common infrastructure for the bootloader and the system flash layout on

- Boot serial: Add response to echo command if support is not

  enabled, previously the command would have been accepted but no

  response indicating that the command is not supported would have

  been sent.

- Added support for using builtin keys for image validation

  (available with the PSA Crypto API based crypto backend for ECDSA signatures).

- Enforce that TLV entries that should be protected are.

  This can be disabled by defining `ALLOW_ROGUE_TLVS`

- bootutil: Fixed issue with comparing sector sizes for

  compatibility, this now also checks against the number of usable

  sectors (which is the slot size minus the swap status and moved

  up by one sector).

- bootutil: Added debug logging to show write location of swap status

  and details on sectors including if slot sizes are not optimal for

  a given board.