Makefile: Use devkit as default target

While imx6 is still nice we want a simple

    make

build a devkit image by default. Use the latest kernel as well
since we're not publishing to storage.puri.sm atm.

Makefile

 # SPDX-License-Identifier: GPL-3.0+
 
-BOARD ?= imx6
-TYPE ?= unstable
+BOARD ?= devkit
+TYPE ?= latest
 DIST ?= buster+ci
 
 SHELL := /bin/bash