lantiq: rework VG3503J image

Use the new image build code and remove the lzma loader. The lzma
loader was used to cheat the signature validation of the bootloader and
I found another way to do this.

To migrate boards already using LEDE/OpenWrt to the new image the
following steps need to be done once:

  VR9 # run reset_uboot_config
  VR9 # reset
  VR9 # setenv ethaddr AA:BB:CC:DD:EE:FF
  VR9 # setenv preboot ping 1.1.1.1\;bootm 0xb001f000
  VR9 # saveenv
  VR9 # tftp 0x81000000 lede-lantiq-xrx200-VG3503J-squashfs-sysupgrade.bin
  VR9 # erase 0xb0020000 $(filesize)
  VR9 # cp.b 0x81000000 0xb0020000 $(filesize)

The mac address is printed on the label at the bottom of the case.

The following steps are need to be done during first install:

  VR9 # setenv preboot ping 1.1.1.1\;bootm 0xb001f000
  VR9 # saveenv
  VR9 # tftp 0x81000000 lede-lantiq-xrx200-VG3503J-squashfs-sysupgrade.bin
  VR9 # erase 0xb0020000 $(filesize)
  VR9 # cp.b 0x81000000 0xb0020000 $(filesize)

The image uses the uImage firmware splitter now instead of hardcoded
kernel and rootfs partitions. The firmware partition size was extended
to use flash space that was reserved for partitions required only by
the ECI firmware.

Due to the changes an upgrade to a later LEDE revision from a running
LEDE is supported now.

A default switch config was added and the device uses the same MAC
addresses as the ECI firmware now instead of the same for all VG3503J.

Signed-off-by: Mathias Kresin <dev@kresin.me>