Apply to mr3420_3220v1/ap99/boot/u-boot/include/configs/ap99.h . Taken from #1 pupie post, fixed formatting and removed device-specific lines https://forum.openwrt.org/viewtopic.php?id=32512

ap99-16MiB.h.patch

--- ap99.h.original	2010-07-05 03:23:40.000000000 +0200
+++ ap99.h.fixed	2015-03-19 03:26:25.515365506 +0100
@@ -6,26 +6,38 @@
 #define __CONFIG_H
 
 #include <configs/ar7240.h>
+
+//Support 8M/16M flash
+#define FLASH_SIZE 16
+
 /*-----------------------------------------------------------------------
  * FLASH and environment organization
  */
 #define CFG_MAX_FLASH_BANKS     1	    /* max number of memory banks */
+#if (FLASH_SIZE == 16)
+#define CFG_MAX_FLASH_SECT      256    /* max number of sectors on one chip, NEED TO CHECK FLASH DATASHEET */
+#else 
 #if (FLASH_SIZE == 8)
 #define CFG_MAX_FLASH_SECT      128    /* max number of sectors on one chip */
 #else
 #define CFG_MAX_FLASH_SECT      64    /* max number of sectors on one chip */
 #endif
+#endif
 #ifdef CONFIG_K31
 #define CFG_FLASH_SECTOR_SIZE   (256*1024)
 #define CFG_FLASH_SIZE          0x1000000 /* Total flash size */
 #else
 #define CFG_FLASH_SECTOR_SIZE   (64*1024)
+#if (FLASH_SIZE == 16)
+#define CFG_FLASH_SIZE          0x01000000 /* Total flash size */
+#else
 #if (FLASH_SIZE == 8)
 #define CFG_FLASH_SIZE          0x00800000 /* Total flash size */
 #else
 #define CFG_FLASH_SIZE          0x00400000 /* Total flash size */
 #endif
 #endif
+#endif
 
 #define ENABLE_DYNAMIC_CONF 1
 #define CONFIG_SUPPORT_AR7241 1
@@ -64,16 +76,29 @@
 
 #undef CONFIG_BOOTARGS
 /* XXX - putting rootfs in last partition results in jffs errors */
-#if (FLASH_SIZE == 8)
-#define CONFIG_BOOTARGS     "console=ttyS0,115200 root=31:02 rootfstype=jffs2 init=/sbin/init mtdparts=ar7240-nor0:256k(u-boot),64k(u-boot-env),5120k(rootfs),896k(uImage),64k(NVRAM),1792k(ART)"
+#if (FLASH_SIZE == 16)
+#define CONFIG_BOOTARGS     "console=ttyS0,115200 root=31:02 rootfstype=jffs2 init=/sbin/init mtdparts=ar7240-nor0:256k(u-boot),64k(u-boot-env),13312k(rootfs),896k(uImage),64k(NVRAM),1792k(ART)"
 #else
-#define	CONFIG_BOOTARGS     "console=ttyS0,115200 root=31:02 rootfstype=jffs2 init=/sbin/init mtdparts=ar7240-nor0:256k(u-boot),64k(u-boot-env),2752k(rootfs),896k(uImage),64k(NVRAM),64k(ART) REVISIONID"
+    #if (FLASH_SIZE == 8)
+        #define CONFIG_BOOTARGS     "console=ttyS0,115200 root=31:02 rootfstype=jffs2 init=/sbin/init mtdparts=ar7240-nor0:256k(u-boot),64k(u-boot-env),5120k(rootfs),896k(uImage),64k(NVRAM),1792k(ART)"
+    #else
+        #define    CONFIG_BOOTARGS     "console=ttyS0,115200 root=31:02 rootfstype=jffs2 init=/sbin/init mtdparts=ar7240-nor0:256k(u-boot),64k(u-boot-env),2752k(rootfs),896k(uImage),64k(NVRAM),64k(ART) REVISIONID"
+    #endif
 #endif
 
 
 /* default mtd partition table */
 #undef MTDPARTS_DEFAULT
+//#define MTDPARTS_DEFAULT    "mtdparts=ar7240-nor0:256k(u-boot),64k(u-boot-env),2752k(rootfs),896k(uImage),64k(NVRAM),64k(ART)"
+#if (FLASH_SIZE == 16)
+#define MTDPARTS_DEFAULT    "mtdparts=ar7240-nor0:256k(u-boot),64k(u-boot-env),13312k(rootfs),896k(uImage),64k(NVRAM),1792k(ART)"
+#else
+#if (FLASH_SIZE == 8)
+#define MTDPARTS_DEFAULT    "mtdparts=ar7240-nor0:256k(u-boot),64k(u-boot-env),5120k(rootfs),896k(uImage),64k(NVRAM),1792k(ART)"
+#else
 #define MTDPARTS_DEFAULT    "mtdparts=ar7240-nor0:256k(u-boot),64k(u-boot-env),2752k(rootfs),896k(uImage),64k(NVRAM),64k(ART)"
+#endif
+#endif
 
 #undef CFG_PLL_FREQ
 
@@ -150,13 +175,18 @@
 #if defined(CONFIG_K31)
 #define CONFIG_BOOTCOMMAND "bootm 0x9f050000"
 #else
+#if (FLASH_SIZE == 16)
+#define CONFIG_BOOTCOMMAND "bootm 0x9f020000"
+#else
 #if (FLASH_SIZE == 8)
-#define CONFIG_BOOTCOMMAND "bootm 0x9f550000"
+//#define CONFIG_BOOTCOMMAND "bootm 0x9f550000"
+#define CONFIG_BOOTCOMMAND "bootm 0x9f020000"
 #else
 //#define CONFIG_BOOTCOMMAND "bootm 0x9f300000"
 #define CONFIG_BOOTCOMMAND "bootm 0x9f020000"
 #endif
 #endif
+#endif
 //#define CONFIG_FLASH_16BIT
 
 /* DDR init values */