doc: remove warning on failed code highlighting

Four code snippets fail to be properly recognized by the code
highlighter; the makefile snippets seem to be because of the $
character (and I have found a way to escape it out). The C code, I
haven't been able to find out why.

Killed the warning by setting the code-block style to 'none' on all of
them. It is not the best sollution, but it beats a polluted build
process.

Change-Id: I8fa7d327354a93bacad40e25596c9dbaf9ea1e92
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>