nxp: Rename CONFIG_U_BOOT_HDR_SIZE to FSL_U_BOOT_HDR_SIZE

This is always defined to 16K, so we move this over to
include/fsl_validate.h to start with. Next, we rename this from CONFIG_
to FSL_. Coalesce the various comments around this definition to be in
fsl_validate.h as well to explain the usage.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/include/configs/T104xRDB.h b/include/configs/T104xRDB.h
index 5bdc210..7a5bf93 100644
--- a/include/configs/T104xRDB.h
+++ b/include/configs/T104xRDB.h
@@ -20,13 +20,7 @@
 
 #ifdef CONFIG_MTD_RAW_NAND
 #ifdef CONFIG_NXP_ESBC
-#define CONFIG_U_BOOT_HDR_SIZE		(16 << 10)
-/*
- * HDR would be appended at end of image and copied to DDR along
- * with U-Boot image.
- */
-#define CFG_SYS_NAND_U_BOOT_SIZE	((768 << 10) + \
-					 CONFIG_U_BOOT_HDR_SIZE)
+#define CFG_SYS_NAND_U_BOOT_SIZE	((768 << 10) + (16 << 10))
 #else
 #define CFG_SYS_NAND_U_BOOT_SIZE	(768 << 10)
 #endif