1009 - Unknown variable '%s'.

Description
A variable which has not been defined was referenced inside another variable assignment or concatenation.
Example
Config:
.Var = '$OtherVar$'
Output:
c:\Test\fbuild.bff(1):(10) FASTBuild Error #1009 - Unknown variable 'OtherVar'. .Var = '$OtherVar$' ^ \--here
Fix:
.OtherVar = 'String' .Var = '$OtherVar$'