ar71xx: WNR2200: fix mounting root fs problem

Solves ticket #14356 <https://dev.openwrt.org/ticket/14356>

Without this patch, UART output will show only 4 partitions:
  [    0.740000] 4 cmdlinepart partitions found on MTD device spi0.0
  [    0.750000] Creating 4 MTD partitions on "spi0.0":
  [    0.750000] 0x000000000000-0x000000050000 : "u-boot"
  [    0.760000] 0x000000050000-0x000000070000 : "u-boot-env"
  [    0.770000] 0x000000070000-0x0000007f0000 : "firmware"
  [    0.790000] 0x0000007f0000-0x000000800000 : "art"
and then linux complains :VFS: Cannot open root device "(null)"

With this patch, UART out will show 7 partitions:
  [    0.740000] 4 cmdlinepart partitions found on MTD device spi0.0
  [    0.750000] Creating 4 MTD partitions on "spi0.0":
  [    0.750000] 0x000000000000-0x000000040000 : "u-boot"
  [    0.760000] 0x000000040000-0x000000050000 : "u-boot-env"
  [    0.770000] 0x000000050000-0x0000007f0000 : "firmware"
  [    0.780000] 2 netgear-fw partitions found on MTD device firmware
  [    0.780000] 0x000000050000-0x00000014b440 : "kernel"
  [    0.790000] mtd: partition "kernel" must either start or end on erase block boundary or be smaller than an erase block -- forcing read-only
  [    0.800000] 0x00000014b440-0x0000007f0000 : "rootfs"
  [    0.810000] mtd: partition "rootfs" must either start or end on erase block boundary or be smaller than an erase block -- forcing read-only
  [    0.820000] mtd: device 4 (rootfs) set to be root filesystem
  [    0.830000] 1 squashfs-split partitions found on MTD device rootfs
  [    0.830000] 0x000000340000-0x0000007f0000 : "rootfs_data"
  [    0.840000] 0x0000007f0000-0x000000800000 : "art"

This patch is tested against r39502 of trunk.

Signed-off-by: Jonathan Chang <changcs@santos.ee.ntu.edu.tw>
Patchwork: http://patchwork.openwrt.org/patch/4870/
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 39625

target/linux/ar71xx/image/Makefile

@@ -234,7 +234,7 @@ ubdev_mtdlayout=mtdparts=spi0.0:256k(u-boot)ro,64k(u-boot-env)ro,7488k(firmware)
 whrhpg300n_mtdlayout=mtdparts=spi0.0:248k(u-boot)ro,8k(u-boot-env)ro,3712k(firmware),64k(art)ro
 wlr8100_mtdlayout=mtdparts=spi0.0:192k(u-boot)ro,64k(u-boot-env)ro,1408k(kernel),14080k(rootfs),192k(unknown)ro,64k(art)ro,384k(unknown2)ro,15488k@0x40000(firmware)
 wndap360_mtdlayout=mtdparts=spi0.0:256k(u-boot)ro,64k(u-boot-env)ro,1728k(kernel),6016k(rootfs),64k(nvram)ro,64k(art)ro,7744k@0x50000(firmware)
-wnr2200_mtdlayout=mtdparts=spi0.0:320k(u-boot)ro,128k(u-boot-env)ro,7680k(firmware),64k(art)ro
+wnr2200_mtdlayout=mtdparts=spi0.0:256k(u-boot)ro,64k(u-boot-env)ro,7808k(firmware),64k(art)ro
 wnr2000v3_mtdlayout=mtdparts=spi0.0:256k(u-boot)ro,64k(u-boot-env)ro,3712k(firmware),64k(art)ro
 wndr3700_mtdlayout=mtdparts=spi0.0:320k(u-boot)ro,128k(u-boot-env)ro,7680k(firmware),64k(art)ro
 wndr3700v2_mtdlayout=mtdparts=spi0.0:320k(u-boot)ro,128k(u-boot-env)ro,15872k(firmware),64k(art)ro