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>