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/loader.c b/boot/bootutil/src/loader.c
index dc6d9cb..95a1572 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -1265,7 +1265,7 @@
 
     switch (swap_type) {
     case BOOT_SWAP_TYPE_NONE:
-#ifdef BOOTUTIL_VALIDATE_SLOT0
+#ifdef MCUBOOT_VALIDATE_SLOT0
         rc = boot_validate_slot(0);
         assert(rc == 0);
         if (rc != 0) {