soc: qcom: mdt_loader: Read hash from firmware blob

Since the split elf blobs will always contain the hash segment, we rely on
the blob file to get the hash rather than assume that it will be present in
the mdt file. This change uses the hash index to read the appropriate elf
blob to get the hash segment.

Change-Id: Iaf37a15f7794d417d01d9eda4a9df772623ae19c
Signed-off-by: Siddharth Gupta <sidgup@codeaurora.org>