Add `flash_area_erased_val` "mock" for Zephyr

For Zephyr assume flash is always erased to 0xff. This allows backward
compatibility of the new flash erased to 0 support.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/zephyr/flash_map_extended.c b/boot/zephyr/flash_map_extended.c
index 7133c99..4f6749b 100644
--- a/boot/zephyr/flash_map_extended.c
+++ b/boot/zephyr/flash_map_extended.c
@@ -67,4 +67,10 @@
     sector->fs_size = page.size;
 
     return rc;
-}
\ No newline at end of file
+}
+
+uint8_t flash_area_erased_val(const struct flash_area *fap)
+{
+    (void)fap;
+    return 0xff;
+}