west: sign.py: fix sign when CONFIG_KERNEL_BIN_NAME is used

This fixes the following build error:
```
zephyr/zephyr.elf', needed by 'zephyr/zephyr.ri', missing and no known
rule to make it
```
This appears when CONFIG_KERNEL_BIN_NAME is used.

Therefore, do not use zephyr.elf since some samples might be called
based on CONFIG_KERNEL_BIN_NAME.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
Signed-off-by: Iuliana Prodan <iuliana.prodan@nxp.com>