qt5base: use -device-option to set mkspec variables

Simplify our custom qmake.conf:
  - Include linux_device_{pre,post}.conf to inherit common device
    configuration.
  - Do not add $(STAGING_DIR)/usr/include to include path. It is
    redundant.

In additon to the simplification of code, this change is a preparation
to solve problems occurring when QMAKE_CXX is prepended with ccache path.

Upstream patch is needed by configure to parse device options correctly.
Another upstream patch is needed to remove ARM specific parts from
linux_device_post.conf.

Signed-off-by: Fatih Aşıcı <fatih.asici@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>