Build: substvars file

If a file named substvars exists along the control file, then it has to be a regular file (or a symbolic link to such,) otherwise an error is raised.

All the variables defined in the substvars file must be defined as <name> = <value> where only <value> can be empty. If the equal character or the name are missing, an error is raised.

VALIDVariable= Value
Variable=Value
Variable=
INVALID= Value
 = Value
=
 =