sim: Rename `SimFlashMap` to `SimMultiFlash`

The name `SimFlashMap` is a bit misleading, as the Map part is more of
an implementation detail when the type really just represents multiple
flash devices.

This is just a rename, but the names `SimFlashMap` and the value
`flashmap` occur in a lot of places in the image module.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/simflash/src/lib.rs b/sim/simflash/src/lib.rs
index cd074c9..7500464 100644
--- a/sim/simflash/src/lib.rs
+++ b/sim/simflash/src/lib.rs
@@ -135,7 +135,7 @@
 
 }
 
-pub type SimFlashMap = HashMap<u8, SimFlash>;
+pub type SimMultiFlash = HashMap<u8, SimFlash>;
 
 impl Flash for SimFlash {
     /// The flash drivers tend to erase beyond the bounds of the given range.  Instead, we'll be