[FS#326] Do not name MD5 SHA256. Its not MD5 when its SHA256

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - hngsvtdu at sharklasers.com (hngsvtdu) 

Attached to Project - LEDE Project
Summary - Do not name MD5 SHA256. Its not MD5 when its SHA256
Task Type - Feature Request
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Low
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - This change and some changes before make use of some strange behavior. 

https://git.lede-project.org/?p=source.git;a=commit;h=a2457722918f602c4d9573c3d617ecc6af616bec

This change is named "tools/upslug2: add SHA256 hash to use mirror tarball."

But adds a line like this:
PKG_MIRROR_MD5SUM:=5c69eae3a2affef8f4f7194cfedfdb9175b04ca433f9ae046c85309b42bdb21e

This is not a "MD5SUM".

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=326