linux-imx

Documentation/serial-console.txt: convert it to ReST markup

- Fix identation for the document title;
- use monotonic fonts for paths;
- use quote blocks where needed;
- adjust spaces to properly format paragraphs;
- use :menuselection: for the menu item;
- add it to the user book.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>