Merge remote-tracking branch 'remotes/elmarco/tags/slirp-pull-request' into staging

slirp: update to latest stable-4.2 branch

# gpg: Signature made Tue 28 Jul 2020 15:30:09 BST
# gpg:                using RSA key 87A9BD933F87C606D276F62DDAE8E10975969CE5
# gpg:                issuer "marcandre.lureau@redhat.com"
# gpg: Good signature from "Marc-André Lureau <marcandre.lureau@redhat.com>" [full]
# gpg:                 aka "Marc-André Lureau <marcandre.lureau@gmail.com>" [full]
# Primary key fingerprint: 87A9 BD93 3F87 C606 D276  F62D DAE8 E109 7596 9CE5

* remotes/elmarco/tags/slirp-pull-request:
  slirp: update to latest stable-4.2 branch
  test-char: abort on serial test error

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