sanitycheck: Fixed documentation for --extra-args and extra_args

sanitycheck was incorrectly documenting that --extra-args would pass
on it's input unchanged to Make.

In reality --extra-args acts as a way to define extra CMake cache
entries. The key-value entries will be prefixed with -D before being
passed to CMake.

E.g
"sanitycheck -x=USE_CCACHE=0"
will translate to
"cmake -DUSE_CCACHE=0"

Signed-off-by: Sebastian Boe <sebastian.boe@nordicsemi.no>