Mynewt: Make PRIMARY/SECONDARY macros public

This change only affects the Mynewt port.

Prior to this commit, the `FLASH_AREA_IMAGE_{PRIMARY,SECONDARY}` macros
were private to the bootutil package (defined in `bootutil_priv.h`).
Some other packages require these symbols (e.g., the boot_serial unit
tests), so they should be made public.

This commit moves these definitions into the public `bootutil.h` header.

Signed-off-by: Christopher Collins <ccollins@apache.org>
diff --git a/boot/bootutil/src/bootutil_priv.h b/boot/bootutil/src/bootutil_priv.h
index b6bb792..dba19c6 100644
--- a/boot/bootutil/src/bootutil_priv.h
+++ b/boot/bootutil/src/bootutil_priv.h
@@ -24,6 +24,7 @@
 
 #include <flash_map_backend/flash_map_backend.h>
 
+#include "bootutil/bootutil.h"
 #include "bootutil/image.h"
 #include "mcuboot_config/mcuboot_config.h"
 
@@ -145,11 +146,6 @@
 #define BOOT_FLAG_IMAGE_OK              0
 #define BOOT_FLAG_COPY_DONE             1
 
-#if defined(MCUBOOT_MYNEWT)
-    #define FLASH_AREA_IMAGE_PRIMARY    FLASH_AREA_IMAGE_0
-    #define FLASH_AREA_IMAGE_SECONDARY  FLASH_AREA_IMAGE_1
-#endif
-
 extern const uint32_t BOOT_MAGIC_SZ;
 
 /**