generic: add pending support for NVMEM ASCII ENV layout driver

generic: add pending support for NVMEM ASCII ENV layout driver

Add pending patch to support NVMEM ASCII ENV layout driver. This is a
generic driver to handle simple NVMEM partition that store environment
in a simple text format. This is the case for Linksys devinfo partition
that are litterally txt file with format "name=value\n"

Such driver works similar to u-boot,env with a similar format. While at
it also introduce a patch to generalize mac-base handling for also other
layout driver.

Link: https://github.com/openwrt/openwrt/pull/17839
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>