cmake: removed stray space in readelf in-/outfile flag

Removed stray whitespace in readelf infile and outfile flag.

Some bintools use `--arg=<val>`, others `--arg <val>` therefore the call
at this location should not force a space, as that will fail on tools
using `=`, as this becomes: `--arg= <val>`

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>