[build] add a few pseudo targets to assist with building

list-arch and list-toolchain will compute and print the architecture and
toolchain needed to build the project the build is set to.

Used by the CI builder to decide what toolchain to grab.

make/help.mk

@@ -23,6 +23,9 @@ help:
 	@echo "make spotless: removes all build directories"
 	@echo "make <project>: try to build project named <project>"
 	@echo ""
+	@echo "make list-arch: print the architecture of the current project"
+	@echo "make list-toolchain: print the computed toolchain prefix of the current project"
+	@echo ""
 	@echo "Examples:"
 	@echo "PROJECT=testproject make"
 	@echo "PROJECT=testproject make clean"