vela: Improve block configuration and weight buffering algorithm

 - Update block config selection to take into account partial
   IFM fetches at edge of non-whole OFM block data.
 - Change to scheduler depth slicing for networks in MLBEDSW-4637
   for improved buffering. This helps general performance by buffering
   larger depth slices.
 - Bug fix for opt_max_schedule always being fitted to SRAM which
   prevented the optimisation step running in some cases.

Signed-off-by: Tim Hall <tim.hall@arm.com>
Change-Id: I97642c5adec3bb684b1daabf2b81574c27d4eef2