ramips: Add support command fw_setsys for Xiaomi routers

ramips: Add support command fw_setsys for Xiaomi routers

The system parameters are contained in the Bdata partition.
To use the fw_setsys command, you need to create a file
fw_sys.config.
This file is created after calling the functions
ubootenv_add_uci_sys_config and ubootenv_add_app_config.

Signed-off-by: Oleg S <remittor@gmail.com>
[ wrapped commit description to 72 char ]
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
(cherry picked from commit 6c7e337c80f92693c2ca628a4a56aeaec4cc3ca8)