[dv,bazel] only copy over an ELF file if one exists
[dv,bazel] only copy over an ELF file if one exists

ELF files are loaded in DV to allow backdoor reading/writing SW symbols.
But in some cases, we do not need to load an ELF, i.e., if we are
testing a corrupted flash image.

Signed-off-by: Timothy Trippel <77eacc84f9735d3cbb55a271710be269fc3a5818@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan