php: add opcache option

Add a Zend OPcache option and tweak the build system into being
cross-compile friendly.
The OPcache extension allows for a neat intermediate bytecode cache in
memory to avoid reinterpreting/recompiling php scripts on multiple runs.
Make it an option since this takes up valuable RAM.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>