ar71xx: hiwifi-hc6361: lift size limit on kernel and rootfs parts

ar71xx: hiwifi-hc6361: lift size limit on kernel and rootfs parts

The patch has been run-tested and the relevant dmsg logs are as the
following

    [    0.762447] Creating 5 MTD partitions on "spi0.0":
    [    0.767217] 0x000000000000-0x000000010000 : "u-boot"
    [    0.775139] 0x000000010000-0x000000020000 : "bdinfo"
    [    0.781014] 0x000000020000-0x000000fe0000 : "firmware"
    [    0.810558] 2 uimage-fw partitions found on MTD device firmware
    [    0.815043] 0x000000020000-0x000000170000 : "kernel"
    [    0.821925] 0x000000170000-0x000000fe0000 : "rootfs"
    [    0.827587] mtd: device 4 (rootfs) set to be root filesystem
    [    0.831937] 1 squashfs-split partitions found on MTD device rootfs
    [    0.837983] 0x0000005c0000-0x000000fe0000 : "rootfs_data"
    [    0.845621] 0x000000fe0000-0x000000ff0000 : "backup"
    [    0.851445] 0x000000ff0000-0x000001000000 : "art"

While at it, convert to new build method

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>