8bit AVR port from Michael Rolnik.

Michael started to work on the AVR port few years ago [*] and kept
improving the code over various series.

List of people who help him (in chronological order):
- Richard Henderson
- Sarah Harris and Edward Robbins
- Philippe Mathieu-Daudé and Aleksandar Markovic
- Pavel Dovgalyuk
- Thomas Huth

[*] The oldest contribution I could find on the list is from 2016:
https://lists.nongnu.org/archive/html/qemu-devel/2016-06/msg02985.html

Tests included:

$ avocado --show=app run -t arch:avr tests/acceptance/
Fetching asset from tests/acceptance/machine_avr6.py:AVR6Machine.test_freertos
 (1/1) tests/acceptance/machine_avr6.py:AVR6Machine.test_freertos: PASS (2.13 s)
RESULTS    : PASS 1 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB TIME   : 2.35 s

$ make check-qtest-avr
  TEST    check-qtest-avr: tests/qtest/boot-serial-test
  TEST    check-qtest-avr: tests/qtest/cdrom-test
  TEST    check-qtest-avr: tests/qtest/device-introspect-test
  TEST    check-qtest-avr: tests/qtest/machine-none-test
  TEST    check-qtest-avr: tests/qtest/qmp-test
  TEST    check-qtest-avr: tests/qtest/qmp-cmd-test
  TEST    check-qtest-avr: tests/qtest/qom-test
  TEST    check-qtest-avr: tests/qtest/test-hmp
  TEST    check-qtest-avr: tests/qtest/qos-test

CI results:
. https://cirrus-ci.com/build/5697049146425344
. https://gitlab.com/philmd/qemu/-/pipelines/165328058
. https://travis-ci.org/github/philmd/qemu/builds/705817933
. https://app.shippable.com/github/philmd/qemu/runs/822/summary/console



# gpg: Signature made Sat 11 Jul 2020 10:03:11 BST
# gpg:                using RSA key FAABE75E12917221DCFD6BB2E3E32C2CDEADC0DE
# gpg: Good signature from "Philippe Mathieu-Daudé (F4BUG) <f4bug@amsat.org>" [full]
# Primary key fingerprint: FAAB E75E 1291 7221 DCFD  6BB2 E3E3 2C2C DEAD C0DE

* remotes/philmd-gitlab/tags/avr-port-20200711: (32 commits)
  target/avr/disas: Fix store instructions display order
  target/avr/cpu: Fix $PC displayed address
  target/avr/cpu: Drop tlb_flush() in avr_cpu_reset()
  target/avr: Add section into QEMU documentation
  tests/acceptance: Test the Arduino MEGA2560 board
  tests/boot-serial: Test some Arduino boards (AVR based)
  hw/avr: Add limited support for some Arduino boards
  hw/avr: Add some ATmega microcontrollers
  hw/avr: Add support for loading ELF/raw binaries
  hw/misc: avr: Add limited support for power reduction device
  hw/timer: avr: Add limited support for 16-bit timer peripheral
  hw/char: avr: Add limited support for USART peripheral
  tests/machine-none: Add AVR support
  target/avr: Register AVR support with the rest of QEMU
  target/avr: Add support for disassembling via option '-d in_asm'
  target/avr: Initialize TCG register variables
  target/avr: Add instruction translation - CPU main translation function
  target/avr: Add instruction translation - MCU Control Instructions
  target/avr: Add instruction translation - Bit and Bit-test Instructions
  target/avr: Add instruction translation - Data Transfer Instructions
  ...

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>