Makefile: use --output-target instead of --target

Makefile: use --output-target instead of --target

The objcopy man-page teaches:

    --target=bfdname
      Use bfdname as the object format for
      both the input and the output file

This implies for --target=efi-app-x86_64 that the input file would have
to be an EFI app.

Objcopy in binutils 2.45 checks this more strictly than previous versions
and refuses to accept an ELF file as input with --target=efi-app-x86_64.

Replace --target by --output-target for building sandbox and x86 EFI
binaries.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>