Add sim support for flash erased at 0

This extends the simulator to be able to test the bootloader in devices
which use flash technologies that erase flash at 0 instead of 0xff.
Two MCU devices that have this "property" are the STM32L0x and STM32L1x
lines from ST.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/sim/mcuboot-sys/csupport/run.c b/sim/mcuboot-sys/csupport/run.c
index ece8555..8238b39 100644
--- a/sim/mcuboot-sys/csupport/run.c
+++ b/sim/mcuboot-sys/csupport/run.c
@@ -52,6 +52,13 @@
     return sim_flash_align;
 }
 
+uint8_t sim_flash_erased_val = 0xff;
+uint8_t flash_area_erased_val(const struct flash_area *area)
+{
+    (void)area;
+    return sim_flash_erased_val;
+}
+
 struct area {
     struct flash_area whole;
     struct flash_area *areas;