sim: define stm32f4 with non-linear sectors slots

Change the flash layout of the stm32f4 simulated device to include
sectors of different sizes inside both slots; both slots have been
updated to still share the same layout. This layout changes allow for
testing and validation that a swap with scratch is possible even if not
all sectors inside a slot are the same size, as long as they all fit in
scratch.

Signed-off-by: Fabio Utzig <utzig@apache.org>