config: Allow btvars to be used as rtvar default values
This allows a single parameter to be specified that can be used at both build-time and run-time. A btvar is defined and set as normal. And a corresponding rtvar is defined with its default value set to the btvar macro. The user can still override the rtvar at runtime if they want.
Example:
buildex: btvars: MYPROP: type: string value: null build: mycomponent: params: myprop: ${btvar:MYPROP} run: rtvars: MYPROP: type: string value: ${btvar:MYPROP} params: -C myprop: ${rtvar:MYPROP}
Signed-off-by: Ryan Roberts ryan.roberts@arm.com