makefile: Avoid objcopy --gap-fill for .hex/.srec

makefile: Avoid objcopy --gap-fill for .hex/.srec

This flag only makes sense for `binary` output, because .hex/.srec are
sparse formats and represent gaps without filler. While the GNU binutils
version of objcopy does not seem to mind the extra flag being passed,
llvm-objcopy considers this a fatal error.

There is already a version of the objcopy command template in the
Makefile that doesn't use --gap-fill, which is provided for EFI. So use
this other version for all .hex/.srec outputs as well.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>