README: Fix evb-ast2500 config

The evb-ast2500 config by `. setup` command was broken due to duplicated
machine config. Add a special note about how to manually specify the
machine config to setup the build.

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: Ifad02bef1c8c5246494adcc3a8a4b6aaa974633b