Merge changes from topic "bk/warnings" into integration

Merge changes from topic "bk/warnings" into integration

* changes:
  docs: describe the new warning levels
  build: add -Wunused-const-variable=2 to W=2
  build: include -Wextra in generic builds
  docs(porting-guide): update a reference
  fix(st-usb): replace redundant checks with asserts
  fix(brcm): add braces around bodies of conditionals
  fix(renesas): align incompatible function pointers
  fix(zynqmp): remove redundant api_version check
  fix: remove old-style declarations
  fix: unify fallthrough annotations