net: openthread: New build options for OpenThread

Some options were available in the options.cmake but were not reflected
in Kconfig.
Added possibility to enable additional configuration options for OT.
Some of the options were left commented out as those options are not
yet supported e.g. require Thread 1.2 or require shim changes.
As openthread has gazillion configuration options that are passed as
define value, created generic option for passing any number of those
values separated with space.

Signed-off-by: Marek Porwisz <marek.porwisz@nordicsemi.no>