Remove boot_swap_type usage in the bootloader

`boot_swap_type_multi` should be used instead. The `BOOT_CURR_IMG` macro
already returns the correct value in single image mode.

For compatibility with external tools like mcumgr, `boot_swap_type`
remains available and always returns the swap type of image 0.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/bootutil/src/bootutil_misc.c b/boot/bootutil/src/bootutil_misc.c
index 46aaa61..1b26636 100644
--- a/boot/bootutil/src/bootutil_misc.c
+++ b/boot/bootutil/src/bootutil_misc.c
@@ -580,13 +580,16 @@
     return BOOT_SWAP_TYPE_NONE;
 }
 
+/*
+ * This function is not used by the bootloader itself, but its required API
+ * by external tooling like mcumgr.
+ */
 int
 boot_swap_type(void)
 {
     return boot_swap_type_multi(0);
 }
 
-
 /**
  * Marks the image in the secondary slot as pending.  On the next reboot,
  * the system will perform a one-time boot of the the secondary slot image.