objcopy: add option to specify custom prefix for symbol of binary input

When using --input-target=binary, objcopy currently derives symbol names
from a mangled version of the input file name.  This approach can lead to
unpredictable results, as the generated symbols depend on the file path and
working directory.

This patch introduces a new option:

  --binary-symbol-prefix <prefix> Use <prefix> as the base symbol name for
                                    the input file (default: derived from
                                    file name)

It allows specifying an explicit symbol prefix, while preserving the existing
behavior as a fallback.

Signed-off-by: Alon Bar-Lev <alon.barlev@gmail.com>