west: runners: blackmagicprobe: elf_file fallback

Building the HEX file is optional (CONFIG_BUILD_OUTPUT_HEX), so
`bmp_flash` will fallback to elf_file if missing. Additionally, to
maintain section names the HEX is only used if it is signed.

Signed-off-by: John Whittington <git@jbrengineering.co.uk>