board: add Orangepi Zero Plus support

Add initial support for Orangepi Zero Plus board:
- U-Boot 2020.10
- Linux 5.7.19
- Default packages from buildroot

This patch is based on Orangepi Zero Plus2 board support, so genimage
configuration and description is copy pasted. One difference is that
newer Linux and U-Boot are selected. Another difference is Ethernet
support (Plus) instead of WiFi (Plus2).

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>