Documentation/sphinx: let the user specify PDFLATEX and LATEXOPTS

Refer to xelatex and latex options via variables. This allows the user
to override the pdflatex and latex options to use on the make command
line for experimenting. As a side effect, this makes the makefile a bit
tidier.

Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>