bitbake-user-manual: Correct description of the ??= operator
    
    Stating that the assignment is done at the end of parsing is misleading.
    The weak default value is the value which a variable will expand to if no value
    has been assigned to it using any of the assignment operators.
    
    Signed-off-by: Jacob Kroon <jacob.kroon@gmail.com>
    Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
    Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>