Ternary operator macro for yaml

Use case is to set multiple values in a config based on a btvar, for e.g. selecting between debug/release:

build: tfa: params: DEBUG: ${btvar:TFA_DEBUG} artifacts: BL1: {param:builddir}/fvp/{btvar:TFA_DEBUG ? debug : release}/bl1.bin buildex: btvars: TFA_DBUG: 0

Need to consider what evaluates to false; 0?, False?, None?, Null?, empty string? (all case insensitive?). Any president we can follow? Do we need to support nesting? I don't think we currently support that.