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_rsa.c b/boot/bootutil/src/image_rsa.c
index 4890f46..3ea1a89 100644
--- a/boot/bootutil/src/image_rsa.c
+++ b/boot/bootutil/src/image_rsa.c
@@ -19,9 +19,7 @@
 
 #include <string.h>
 
-#include "syscfg/syscfg.h"
-
-#if MYNEWT_VAL(BOOTUTIL_SIGN_RSA)
+#ifdef MCUBOOT_SIGN_RSA
 #include "bootutil/sign_key.h"
 
 #include "mbedtls/rsa.h"
@@ -143,4 +141,4 @@
 
     return rc;
 }
-#endif /* MYNEWT_VAL(BOOTUTIL_SIGN_RSA) */
+#endif /* MCUBOOT_SIGN_RSA */