openwrt中Makefile函数subset分析

假设此处$(1) =target，
则

target/builddirs = linux sdk imagebuilder toolchain
SUBTARGETS = clean download prepare compile install update refresh prereq dist distcheck configure
buildtypes-$(bd) =

# Parameters: <subdir>
define subdir
  $(call warn,$(1),d,D $(1))
  $(foreach bd,$($(1)/builddirs),
    $(call warn,$(1),d,BD $(1)/$(bd))
    $(foreach target,$(SUBTARGETS),
      $(foreach btype,$(buildtypes-$(bd)),
        $(call warn_eval,$(1)/$(bd),t,T,$(1)/$(bd)/$(btype)/$(target): $(if $(QUILT),,$($(1)/$(bd)/$(btype)/$(target)) $(call $(1)//$(btype)/$(target),$(1)/$(bd)/$(btype))))
          $(if $(call debug,$(1)/$(bd),v),,@)+$$(SUBMAKE) -C $(1)/$(bd) $(btype)-$(target) $(if $(findstring $(bd),$($(1)/builddirs-ignore-$(btype)-$(target))), || $(call ERROR,$(1),   ERROR: $(1)/$(bd) [$(btype)] failed to build.))
        $(if $(call diralias,$(bd)),$(call warn_eval,$(1)/$(bd),l,T,$(1)/$(call diralias,$(bd))/$(btype)/$(target): $(1)/$(bd)/$(btype)/$(target)))
      )
      $(call warn_eval,$(1)/$(bd),t,T,$(1)/$(bd)/$(target): $(if $(QUILT),,$($(1)/$(bd)/$(target)) $(call $(1)//$(target),$(1)/$(bd))))
	  	$(if $(BUILD_LOG),@mkdir -p $(BUILD_LOG_DIR)/$(1)/$(bd))
        $(foreach variant,$(if $(BUILD_VARIANT),$(BUILD_VARIANT),$(if $(strip $($(1)/$(bd)/variants)),$($(1)/$(bd)/variants),$(if $($(1)/$(bd)/default-variant),$($(1)/$(bd)/default-variant),__default))),
			$(if $(call debug,$(1)/$(bd),v),,@)+$(if $(BUILD_LOG),set -o pipefail;) $$(SUBMAKE) -C $(1)/$(bd) $(target) BUILD_VARIANT="$(filter-out __default,$(variant))" $(if $(BUILD_LOG),SILENT= 2>&1 | tee $(BUILD_LOG_DIR)/$(1)/$(bd)/$(target).txt) $(if $(findstring $(bd),$($(1)/builddirs-ignore-$(target))), || $(call ERROR,$(1),   ERROR: $(1)/$(bd) failed to build$(if $(filter-out __default,$(variant)), (build variant: $(variant))).))
        )
      $(if $(PREREQ_ONLY)$(DUMP_TARGET_DB),,
        # aliases
        $(if $(call diralias,$(bd)),$(call warn_eval,$(1)/$(bd),l,T,$(1)/$(call diralias,$(bd))/$(target): $(1)/$(bd)/$(target)))
      )
    )
  )
  $(foreach target,$(SUBTARGETS),$(call subtarget,$(1),$(target)))
endef