Build fx subcommands

Command Description
args manually edit for the current build directory
build Run Ninja to build Fuchsia
clean clean the current Fuchsia and Zircon build artifacts
clean-build clean the build directory and then perform a full build
gen run gn gen against the Zircon and Fuchsia build directories.
get-build-dir print the current fuchsia build directory
gn meta-build system that generates build files for Ninja
list-boards list boards available for building
list-packages list packages are built
list-products list products available for building
ninja directly run the build system. Don't use for regular workflows.
ninjatrace2json Collect Ninja trace information for analysis in chrome://tracing
set set up a build directory
set-build-dir set the default build directory used by other fx commands
use re-use a previous build directory set up by fx set
check-deps checks dependency graph in areas
compdb generate a compilation database for the current build configuration
verify-build-packages verify the structure of the build package directory in a layer