[BUILD] Allow configuration of compiler optimisation in Kconfig.

* Fixed some nasty gotchas when starting a FW project, default includes are now automatic.
* CPU architectures must now specify their actual -mcpu="cpu-name".
* TOOLCHAIN is now automatically created based on the compiler prefix found in Kconfig.
[CONFIG] Added support for BCM2836 to specify its -mcpu flags correctly.
[CONFIG] Allow configuration of debug symbols during build.
[BSP] RPi - Removed unecessary dbuild.config.mk file.