zephyr: Bring in the Zephyr build

Add support for building mcuboot as a Zephyr application.  This is
copied from the iotboot repo with some minor reorganization to work with
the mcuboot directory layout.
diff --git a/zephyr/build_boot.sh b/zephyr/build_boot.sh
new file mode 100755
index 0000000..2fc8326
--- /dev/null
+++ b/zephyr/build_boot.sh
@@ -0,0 +1,6 @@
+#! /bin/bash
+
+source $(dirname 0)/target.sh
+source ../../zephyr/zephyr-env.sh
+
+make BOARD=$BOARD "$@"