twister: support namespacing of extra configs

We want to be able to have platform or architecture extra configs
without having to duplicate a whole section of the test specification.

This adds support for namespacing of extra configs, for example:

arch:nios2:CONFIG_SYS_CLOCK_TICKS_PER_SEC=1000

or

platform:qemu_x86:CONFIG_FOO=y

Signed-off-by: Anas Nashif <anas.nashif@intel.com>