According to the installation guide [1], the support for "sha256_password"
or "caching_sha2_password" for authentication is optional.

This change makes it optional for the OpenWrt package by providing a build
option to enable it.
By default it won't be enabled, and packages can choose to enable it.

[1] https://pymysql.readthedocs.io/en/latest/user/installation.html

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>