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