zephyr: port build system to CMake

Convert the Zephyr build to the new CMake-based sytem.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>
diff --git a/samples/zephyr/build-boot.sh b/samples/zephyr/build-boot.sh
index 20e61a0..81691e7 100755
--- a/samples/zephyr/build-boot.sh
+++ b/samples/zephyr/build-boot.sh
@@ -19,4 +19,4 @@
     die "Please set BOARD to a valid board before running this script."
 fi
 
-make -C ../.. BOARD=${BOARD} -j$(nproc) || die "Build mcuboot"
+make BOARD=${BOARD} -j$(nproc) boot || die "Build mcuboot"