make memsys config global
undo per-transaction burst length support
DDR3 only supports 8-burst transactions, previous DDRs support 2/4/8 but burst length is likely to be fixed at compile-time anyway.

This undoes the following commits:
41d33339a017fe24226126058f01659a74cf1f0e
c93f10f21ed96b0b9b3bd0cc51a27a22d8596530
4f8f3a9f8cff5d3dba1263f7b56737f59de88efe