zephyr: Move Zephyr build to top level

The Zephyr build system does strange things if it refers to source
directories above the main Makefile (generating
outdir/<target>/../boot/... paths).  For now, fix this by moving the
Zephyr Makefile to the top-level in the tree.  This shouldn't have much
impact on Mynewt, as it doesn't use makefiles.
diff --git a/build_boot.sh b/build_boot.sh
new file mode 100755
index 0000000..b998812
--- /dev/null
+++ b/build_boot.sh
@@ -0,0 +1,6 @@
+#! /bin/bash
+
+source $(dirname 0)/target.sh
+source ../zephyr/zephyr-env.sh
+
+make BOARD=$BOARD "$@"