ipq806x: convert TP-Link Archer VR2600v to denx,uimage

The recent device-tree modification that added pre-cal
nvmem-cells pushed the device's kernel+dtb over the
allotted 3072k KERNEL_SIZE.

> WARNING: Image file tplink_vr2600v-uImage is too big: 3147214 > 3145728

There was a previous kernel partition size upgrade:
commit 0c967d92b3 ("ipq806x: increase kernel partition size for the TP-Link Archer VR2600v")
It has been seemingly upgraded from a 2048k KERNEL_SIZE in the past.
The commit talks about using the MTD_SPLIT_TPLINK_FW. But looking at
the image make recipe, there is no code that adds a TPLINK header.
So instead the board will use "denx,umimage". This requires
MTD_SPLIT_UIMAGE_FW, but this is present thanks to some NEC devices.

(Maybe the MTD_CONFIG_ARGS can be removed as well? But it could be
there because of the padding at the beginning. This needs testing.)

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

target/linux/ipq806x/base-files/lib/upgrade/platform.sh

@@ -57,7 +57,6 @@ platform_do_upgrade() {
 		default_do_upgrade "$1"
 		;;
 	tplink,vr2600v)
-		PART_NAME="kernel:rootfs"
 		MTD_CONFIG_ARGS="-s 0x200000"
 		default_do_upgrade "$1"
 		;;