Add RMII example

examples/rp2040/pico-rmii/Makefile

@@ -0,0 +1,16 @@
+SDK_VERSION ?= 1.4.0
+SDK_REPO ?= https://github.com/raspberrypi/pico-sdk
+
+all example:
+	true
+
+build: pico-sdk
+	test -d build || mkdir build
+	cd build && cmake .. && make
+
+pico-sdk:
+	git clone --depth 1 -b $(SDK_VERSION) $(SDK_REPO) $@
+	cd $@ && git submodule update --init
+
+clean:
+	rm -rf pico-sdk build