Merge remote-tracking branch 'remotes/philmd-gitlab/tags/fw_cfg-20200721' into staging

fw_cfg patches

Fixes the DEADCODE issue reported by Coverity (CID 1430396).

CI jobs result:
. https://gitlab.com/philmd/qemu/-/pipelines/169086301

# gpg: Signature made Tue 21 Jul 2020 18:52:46 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/fw_cfg-20200721:
  hw/nvram/fw_cfg: Let fw_cfg_add_from_generator() return boolean value
  hw/nvram/fw_cfg: Simplify fw_cfg_add_from_generator() error propagation

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