Mynewt: Fix boot_serial unit tests

These unit tests were failing with build errors such as this:

    boot/boot_serial/test/src/testcases/boot_serial_img_msg.c:64:26: error: use of undeclared identifier 'FLASH_AREA_IMAGE_PRIMARY'
        rc = flash_area_open(FLASH_AREA_IMAGE_PRIMARY, &fap);
                             ^
The `FLASH_AREA_IMAGE_{PRIMARY,SECONDARY} definitions were not visible
because `MCUBOOT_MYNEWT` was not getting defined in the boot_serial
unit test package.

This commit defines `MCUBOOT_MYNEWT` in the boot_serial unit test
package, and adds the necessary include to pull in the PRIMARY /
SECONDARY definitions.

Signed-off-by: Christopher Collins <ccollins@apache.org>
diff --git a/boot/boot_serial/test/pkg.yml b/boot/boot_serial/test/pkg.yml
index c2de667..d0b91f7 100644
--- a/boot/boot_serial/test/pkg.yml
+++ b/boot/boot_serial/test/pkg.yml
@@ -30,3 +30,6 @@
 
 pkg.deps.SELFTEST:
     - "@apache-mynewt-core/sys/console/stub"
+
+pkg.cflags:
+    - '-DMCUBOOT_MYNEWT'
diff --git a/boot/boot_serial/test/src/boot_test.h b/boot/boot_serial/test/src/boot_test.h
index 402d5b1..2139d9f 100644
--- a/boot/boot_serial/test/src/boot_test.h
+++ b/boot/boot_serial/test/src/boot_test.h
@@ -33,11 +33,12 @@
 #include "testutil/testutil.h"
 #include "hal/hal_flash.h"
 #include "flash_map_backend/flash_map_backend.h"
+#include "bootutil/bootutil.h"
 
 #include "boot_serial_priv.h"
 
 #ifdef __cplusplus
-#extern "C" {
+extern "C" {
 #endif
 
 void tx_msg(void *src, int len);