doc: extensions: doxyrunner: add doxyrunner_outdir_var option

The doxyrunner_outdir_var option allows to specify which variable (if
any) is used to represent the output directory (as used by
OUTPUT_DIRECTORY). This makes sure that other options referencing it are
processed correctly, since the output directory is not passed as a
variable.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>