mcuboot: flashes but no output

[fjmolinas]

Description

Following a discussion on pthe forum](https://forum.riot-os.org/t/myboot-mcuboot-ported-to-riot/3483), I tested mcuboot on the default hardware nrf52dk. I find that I can flash the application but nothing happens thereafter. This seems to have been the case at least since #11083 (before that building in docker was not possible).

If a try an earlier commit I get a compilation e764678 ERROR:

atomic_c11.c:149:10: error: conflicting types for built-in function '__atomic_nand_fetch_8'; expected 'long long unsigned int(volatile void *, long long unsigned int,  int)' [-Werror=builtin-declaration-mismatch]
  149 |     I##n __atomic_##opname##_fetch_##n(I##n *ptr, I##n val, int memmodel) \
      |          ^~~~~~~~~
atomic_c11.c:238:1: note: in expansion of macro 'TEMPLATE_ATOMIC_OP_FETCH_N'
  238 | TEMPLATE_ATOMIC_OP_FETCH_N(nand, &, 8, ~) /* __atomic_nand_fetch_8 */

If I ignore the errors then it builds and flashes but nothing happens. Same goes when testing on 7358e16.

Steps to reproduce the issue

1. Build and flash mcuboot test application

make -C tests/mcuboot/ mcuboot-flash term

Expected results

Prints

**2022-01-17 10:13:46,840 - INFO # Hello MCUBoot!
2022-01-17 10:13:46,842 - INFO # You are running RIOT on a(n) nrf52dk board.
2022-01-17 10:13:46,846 - INFO # This board features a(n) nrf52 MCU.
2022-01-17 10:13:46,849 - INFO # The startup address is: 0x8200

Actual results

Flashes but nothing else happens

Versions

Operating System Environment
----------------------------
         Operating System: "Ubuntu" "20.04.1 LTS (Focal Fossa)"
                   Kernel: Linux 5.13.0-1017-oem x86_64 x86_64
             System shell: /usr/bin/dash (probably dash)
             make's shell: /usr/bin/dash (probably dash)

Installed compiler toolchains
-----------------------------
               native gcc: gcc (Ubuntu 10.3.0-1ubuntu1) 10.3.0
        arm-none-eabi-gcc: arm-none-eabi-gcc (GNU Arm Embedded Toolchain 10-2020-q4-major) 10.2.1 20201103 (release)
                  avr-gcc: missing
         mips-mti-elf-gcc: missing
           msp430-elf-gcc: missing
       riscv-none-elf-gcc: missing
  riscv64-unknown-elf-gcc: missing
     riscv-none-embed-gcc: missing
     xtensa-esp32-elf-gcc: missing
   xtensa-esp8266-elf-gcc: missing
                    clang: Ubuntu clang version 12.0.0-3ubuntu1~21.04.2

Installed compiler libs
-----------------------
     arm-none-eabi-newlib: "3.3.0"
      mips-mti-elf-newlib: missing
        msp430-elf-newlib: missing
    riscv-none-elf-newlib: missing
riscv64-unknown-elf-newlib: missing
  riscv-none-embed-newlib: missing
  xtensa-esp32-elf-newlib: missing
xtensa-esp8266-elf-newlib: missing
                 avr-libc: missing (missing)

Installed development tools
---------------------------
                   ccache: ccache version 4.2
                    cmake: cmake version 3.18.4
                 cppcheck: Cppcheck 2.3
                  doxygen: 1.9.1
                      git: git version 2.30.2
                     make: GNU Make 4.3
                  openocd: Open On-Chip Debugger 0.11.0+dev-00282-gae6de2f93-dirty (2021-11-19-15:06)
                   python: Python 3.9.5
                  python2: Python 2.7.18
                  python3: Python 3.9.5
                   flake8: error: /home/francisco/.virtualenvs/mcuboot/bin/python3: No module named flake8
               coccinelle: spatch version 1.1.1-00068-g49826315 compiled with OCaml version 4.08.1