ramips: add support for read/write uboot env to Asus RX-AX53U

ramips: add support for read/write uboot env to Asus RX-AX53U

Add support for read/writing uboot env by renaming the second partition
to its stock label "nvram" and remove the deemed unnecessary
"read-only". Split the first partition "u-boot" in two, in order
to allow `fw_setenv` safe write-access to the uboot environment
variables.

This implements hauke's request from [1].
Based on the patch provided by Shiji Yang.

[1] https://github.com/openwrt/openwrt/pull/10400#discussion_r945153224

Co-Authored-By: Shiji Yang <yangshiji66@qq.com>
Signed-off-by: Felix Baumann <felix.bau@gmx.de>
[ improve commit title and description, fix some whitespace problem ]
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>