ramips: improve support for SIM SIMAX1800T and Haier HAR-20S2U1

ramips: improve support for SIM SIMAX1800T and Haier HAR-20S2U1

1. Use ARTIFACTS to build factory image. This change allows users to
   generate initramfs factory image using OpenWrt ImageBuilder.

2. Override the default bootargs property defined in "mt7621.dtsi".
   Although we use the "bootargs-override" property to set bootargs,
   the default "bootargs" property will still be written into the
   device tree, so it is better to override it.

Tested on SIM SIMAX1800T

Signed-off-by: Shiji Yang <yangshiji66@qq.com>