Make mcuboot less mynewt style

Remove most of mynewt specific stuff to a separate port package. This
should make mcuboot less "mynewt'y" and slightly easier to port to.

- Mynewt specific stuff moved to boot/mynewt.
- Sample app moved from apps/boot to boot/mynewt.
- Use MYNEWT_VAL macro only on mynewt port.
- BOOTUTIL_* and MYNEWT_VAL() usage moved to MCUBOOT_ defines.
diff --git a/boot/bootutil/src/image_ec256.c b/boot/bootutil/src/image_ec256.c
index 1e7354e..615fb9f 100644
--- a/boot/bootutil/src/image_ec256.c
+++ b/boot/bootutil/src/image_ec256.c
@@ -19,9 +19,7 @@
 
 #include <string.h>
 
-#include "syscfg/syscfg.h"
-
-#if MYNEWT_VAL(BOOTUTIL_SIGN_EC256)
+#ifdef MCUBOOT_SIGN_EC256
 #include "bootutil/sign_key.h"
 
 #include "mbedtls/oid.h"
@@ -176,4 +174,4 @@
         return -2;
     }
 }
-#endif /* MYNEWT_VAL(BOOTUTIL_SIGN_EC256) */
+#endif /* MCUBOOT_SIGN_EC256 */