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_validate.c b/boot/bootutil/src/image_validate.c
index 0e78407..9413d31 100644
--- a/boot/bootutil/src/image_validate.c
+++ b/boot/bootutil/src/image_validate.c
@@ -22,17 +22,16 @@
 #include <inttypes.h>
 #include <string.h>
 
-#include "syscfg/syscfg.h"
 #include "hal/hal_flash.h"
 #include "flash_map/flash_map.h"
 #include "bootutil/image.h"
 #include "bootutil/sha256.h"
 #include "bootutil/sign_key.h"
 
-#if MYNEWT_VAL(BOOTUTIL_SIGN_RSA)
+#ifdef MCUBOOT_SIGN_RSA
 #include "mbedtls/rsa.h"
 #endif
-#if MYNEWT_VAL(BOOTUTIL_SIGN_EC) || MYNEWT_VAL(BOOTUTIL_SIGN_EC256)
+#if defined(MCUBOOT_SIGN_EC) || defined(MCUBOOT_SIGN_EC256)
 #include "mbedtls/ecdsa.h"
 #endif
 #include "mbedtls/asn1.h"
@@ -96,8 +95,8 @@
     uint32_t off;
     uint32_t size;
     uint32_t sha_off = 0;
-#if MYNEWT_VAL(BOOTUTIL_SIGN_RSA) || MYNEWT_VAL(BOOTUTIL_SIGN_EC) || \
-    MYNEWT_VAL(BOOTUTIL_SIGN_EC256)
+#if defined(MCUBOOT_SIGN_RSA) || defined(MCUBOOT_SIGN_EC) || \
+    defined(MCUBOOT_SIGN_EC256)
     uint32_t sig_off = 0;
     uint32_t sig_len = 0;
 #endif
@@ -106,17 +105,17 @@
     uint8_t hash[32];
     int rc;
 
-#if MYNEWT_VAL(BOOTUTIL_SIGN_RSA)
+#ifdef MCUBOOT_SIGN_RSA
     if ((hdr->ih_flags & IMAGE_F_PKCS15_RSA2048_SHA256) == 0) {
         return -1;
     }
 #endif
-#if MYNEWT_VAL(BOOTUTIL_SIGN_EC)
+#ifdef MCUBOOT_SIGN_EC
     if ((hdr->ih_flags & IMAGE_F_ECDSA224_SHA256) == 0) {
         return -1;
     }
 #endif
-#if MYNEWT_VAL(BOOTUTIL_SIGN_EC256)
+#ifdef MCUBOOT_SIGN_EC256
     if ((hdr->ih_flags & IMAGE_F_ECDSA256_SHA256) == 0) {
         return -1;
     }
@@ -150,7 +149,7 @@
             }
             sha_off = off + sizeof(tlv);
         }
-#if MYNEWT_VAL(BOOTUTIL_SIGN_RSA)
+#ifdef MCUBOOT_SIGN_RSA
         if (tlv.it_type == IMAGE_TLV_RSA2048) {
             if (tlv.it_len != 256) { /* 2048 bits */
                 return -1;
@@ -159,7 +158,7 @@
             sig_len = tlv.it_len;
         }
 #endif
-#if MYNEWT_VAL(BOOTUTIL_SIGN_EC)
+#ifdef MCUBOOT_SIGN_EC
         if (tlv.it_type == IMAGE_TLV_ECDSA224) {
             if (tlv.it_len < 64) { /* oids + 2 * 28 bytes */
                 return -1;
@@ -168,7 +167,7 @@
             sig_len = tlv.it_len;
         }
 #endif
-#if MYNEWT_VAL(BOOTUTIL_SIGN_EC256)
+#ifdef MCUBOOT_SIGN_EC256
         if (tlv.it_type == IMAGE_TLV_ECDSA256) {
             if (tlv.it_len < 72) { /* oids + 2 * 32 bytes */
                 return -1;
@@ -193,8 +192,8 @@
             return -1;
         }
     }
-#if MYNEWT_VAL(BOOTUTIL_SIGN_RSA) || MYNEWT_VAL(BOOTUTIL_SIGN_EC) || \
-    MYNEWT_VAL(BOOTUTIL_SIGN_EC256)
+#if defined(MCUBOOT_SIGN_RSA) || defined(MCUBOOT_SIGN_EC) || \
+    defined(MCUBOOT_SIGN_EC256)
     if (!sig_off) {
         /*
          * Header said there should be PKCS1.v5 signature, no TLV