build: add directory-oriented make workflows

firmware/boards/Makefile

@@ -0,0 +1,44 @@
+override SHELL := /bin/sh
+override MAKEFLAGS += --no-print-directory
+
+.DEFAULT_GOAL := help
+.DELETE_ON_ERROR:
+
+override CARGO := cargo
+override CARGO_FLAGS := --locked --offline
+
+override EMBEDDED_TARGET := thumbv7em-none-eabihf
+override TARGET_DIR := ../target/$(EMBEDDED_TARGET)/release
+override SCRIPTS_DIR := ../scripts
+
+override BOARDS := geprc-f722-v2 kakute-f7-mini-v3
+override BOARD_PACKAGES := $(addprefix kaptan-,$(BOARDS))
+override BOARD_PACKAGE_ARGS := $(addprefix -p ,$(BOARD_PACKAGES))
+
+override PRIMARY_GOAL := $(firstword $(MAKECMDGOALS))
+
+run:
+	@exec python3 ../../scripts/make-argv.py exec run $(CARGO) build --release \
+		$(CARGO_FLAGS) $(BOARD_PACKAGE_ARGS) --target $(EMBEDDED_TARGET)
+	@set -e; for board in $(BOARD_PACKAGES); do \
+		$(SCRIPTS_DIR)/create-dfu-image.sh $(TARGET_DIR)/$$board; \
+	done
+
+ifeq ($(PRIMARY_GOAL),run)
+
+.PHONY: run
+
+# Application arguments may also look like Make goals. The run recipe recovers
+# their exact boundaries from the original invocation.
+.DEFAULT:
+	@:
+
+else
+
+.PHONY: help run
+
+help:
+	@printf '%s\n' 'Usage: make firmware/boards -- <cargo build arguments...>' '' \
+		'With no arguments, every board is built and packaged.'
+
+endif