linux-imx

README: convert it to ReST markup

Adjust the readme file for it to use the ReST markup:

- add chapter/section markups;
- use ``foo`` for commands;
- use :: for verbatim and script blocks;
- replace unsupported markup _foo_ by **foo**;
- add cross-references to other ReST files;
- use lower case on the section titles, to match other ReST files.

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