[rp20xx][pico] platform and target for rp20xx and pico board

- just a skeleton to get us started
- cpu feature config and irq names / vectab2 setup done
- pulled in the second stage for pico as a blob for now
- seems to be hard faulting out of systick setup in platform_init()

project/pico-test.mk

@@ -0,0 +1,11 @@
+LOCAL_DIR := $(GET_LOCAL_DIR)
+
+TARGET := pico
+
+MODULES += \
+	app/shell \
+	app/stringtests \
+	app/tests \
+	lib/cksum \
+	lib/debugcommands \
+