west: runners: blackmagicprobe: flash hex_file

The cfg.elf_file is not signed, so `west flash` with a sysbuild will
not run the application if MCUboot image signature checking is on. Using
the cfg.hex_file for `bmp_flash` resolves since as the signed hex is
used when using sysbuild. Symbols are not required for flashing so the
switching from elf to hex is not an issue.

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