build: Add make install

Following coreos/ignition-dracut#106
add a default `make install` rule that honors `DESTDIR`.  Main
motivation is hacking on ignition in a unified way with ostree,
ignition-dracut and other projects using coreos-assembler.

Makefile

@@ -1,9 +1,24 @@
 export GO111MODULE=on
 
+# Canonical version of this in https://github.com/coreos/coreos-assembler/blob/6eb97016f4dab7d13aa00ae10846f26c1cd1cb02/Makefile#L19
+GOARCH:=$(shell uname -m)
+ifeq ($(GOARCH),x86_64)
+	GOARCH=amd64
+else ifeq ($(GOARCH),aarch64)
+	GOARCH=arm64
+endif
+
 .PHONY: all
 all:
 	./build
 
+# This currently assumes you're using https://github.com/coreos/ignition-dracut/
+# If in the future any other initramfs integration appears, feel free to add a PR
+# to make this configurable.
+.PHONY: install
+install: all
+	install -m 0755 -D -t $(DESTDIR)/usr/lib/dracut/modules.d/30ignition-dracut bin/$(GOARCH)/ignition
+
 .PHONY: vendor
 vendor:
 	@go mod vendor