Merge remote-tracking branch 'remotes/bonzini-gitlab/tags/for-upstream' into staging

meson fixes:
* bump submodule to 0.55.1
* SDL, pixman and zlib fixes
* firmwarepath fix
* fix firmware builds

meson related:
* move install to Meson
* move NSIS to Meson
* do not make meson use cmake
* add description to options

# gpg: Signature made Tue 01 Sep 2020 17:11:03 BST
# gpg:                using RSA key F13338574B662389866C7682BFFBD25F78C7AE83
# gpg:                issuer "pbonzini@redhat.com"
# gpg: Good signature from "Paolo Bonzini <bonzini@gnu.org>" [full]
# gpg:                 aka "Paolo Bonzini <pbonzini@redhat.com>" [full]
# Primary key fingerprint: 46F5 9FBD 57D6 12E7 BFD4  E2F7 7E15 100C CD36 69B1
#      Subkey fingerprint: F133 3857 4B66 2389 866C  7682 BFFB D25F 78C7 AE83

* remotes/bonzini-gitlab/tags/for-upstream: (26 commits)
  Makefile: Fix in-tree clean/distclean
  Makefile: Add back TAGS/ctags/cscope rules
  meson: add description to options
  build: fix recurse-all target
  meson: use pkg-config method to find dependencies
  configure: do not include ${prefix} in firmwarepath
  meson: add pixman dependency to UI modules
  meson: add pixman dependency to chardev/baum module
  meson: add NSIS building
  meson: use meson mandir instead of qemu_mandir
  meson: pass docdir option
  meson: use meson datadir instead of qemu_datadir
  meson: pass qemu_suffix option
  configure: build docdir like other suffixed directories
  configure: always /-seperate directory from qemu_suffix
  configure: rename confsuffix option
  meson: move zlib detection to meson
  build-sys: remove install target from Makefile
  meson: install $localstatedir/run for qga
  meson: install desktop file
  ...

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