Fix Mynewt boot_serial unittest

The boot_serial test was failing build because it hadn't been updated to
use new multi-image flash area nomenclature. This fixes the build.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/mynewt/flash_map_backend/pkg.yml b/boot/mynewt/flash_map_backend/pkg.yml
index 5f04e29..ba1bf5b 100644
--- a/boot/mynewt/flash_map_backend/pkg.yml
+++ b/boot/mynewt/flash_map_backend/pkg.yml
@@ -11,3 +11,4 @@
 
 pkg.deps:
     - "@apache-mynewt-core/sys/flash_map"
+    - "@mcuboot/boot/mynewt/mcuboot_config"