boot/nxboot: add flush barriers and CRC-validate primary before boot

Two hardening fixes for nxboot power-loss resilience:

1. Add flash_partition_flush() calls between critical partition
   operations in perform_update(). Without explicit flush barriers,
   writes may remain buffered in RAM (e.g. via FTL rwbuffer) when
   nxboot proceeds to the next phase. A power loss between phases
   can leave the recovery image uncommitted while the staging
   partition has already been consumed.

   Flush points added:
   - After copy_partition(primary, recovery) completes
   - After copy_partition(update, primary) completes, before
     erasing the staging first sector

2. Replace validate_image_header() with validate_image() in the
   final primary validation path of nxboot_perform_update(). The
   header-only check validates magic and platform identifier but
   does not CRC-check the image body. After an interrupted update,
   a corrupt primary with an intact header would pass this check
   and be booted, resulting in a persistent boot failure.

Signed-off-by: Neil Berkman <neil@xuku.com>

boot/nxboot/loader/boot.c

@@ -417,6 +417,7 @@ static int perform_update(struct nxboot_state *state, bool check_only)
           syslog(LOG_INFO, "Creating recovery image.\n");
           nxboot_progress(nxboot_progress_start, recovery_create);
           copy_partition(primary, recovery, state, false);
+          flash_partition_flush(recovery);
           nxboot_progress(nxboot_progress_end);
           nxboot_progress(nxboot_progress_start, validate_recovery);
           successful = validate_image(recovery);
@@ -444,6 +445,8 @@ static int perform_update(struct nxboot_state *state, bool check_only)
           nxboot_progress(nxboot_progress_start, update_from_update);
           if (copy_partition(update, primary, state, true) >= 0)
             {
+              flash_partition_flush(primary);
+
               /* Erase the first sector of update partition. This marks the
                * partition as updated so we don't end up in an update loop.
                * The sector is written back again during the image
@@ -919,8 +922,7 @@ int nxboot_perform_update(bool check_only)
       return ERROR;
     }
 
-  get_image_header(primary, &header);
-  if (!validate_image_header(&header))
+  if (!validate_image(primary))
     {
       ret = ERROR;
     }

boot/nxboot/loader/flash.c

@@ -76,6 +76,26 @@ int flash_partition_open(const char *path)
   return fd;
 }
 
+/****************************************************************************
+ * Name: flash_partition_flush
+ *
+ * Description:
+ *   Flushes any buffered writes to the underlying storage. This ensures
+ *   data is physically committed to flash before the caller proceeds.
+ *
+ * Input parameters:
+ *   fd: Valid file descriptor.
+ *
+ * Returned Value:
+ *   0 on success, -1 on failure.
+ *
+ ****************************************************************************/
+
+int flash_partition_flush(int fd)
+{
+  return fsync(fd);
+}
+
 /****************************************************************************
  * Name: flash_partition_close
  *

boot/nxboot/loader/flash.h

@@ -79,6 +79,22 @@ int flash_partition_open(const char *path);
 
 int flash_partition_close(int fd);
 
+/****************************************************************************
+ * Name: flash_partition_flush
+ *
+ * Description:
+ *   Flushes any buffered writes to the underlying storage.
+ *
+ * Input parameters:
+ *   fd: Valid file descriptor.
+ *
+ * Returned Value:
+ *   0 on success, -1 on failure.
+ *
+ ****************************************************************************/
+
+int flash_partition_flush(int fd);
+
 /****************************************************************************
  * Name: flash_partition_write
  *