rust-common.bbclass: soft assignment for RUSTLIB path

rust-common.bbclass: soft assignment for RUSTLIB path

As a user i want to override `RUSTLIB` path on a bbclass, lets
call it `XYZ.bbclass`.

If a certain recipe inherits `cargo.bbclass` and `XYZ.bbclass` the
value of `RUSTLIB` is dependent on the order of the inherit.

If `cargo.bbclass` is inherit before `XYZ.bbclass` this will reflect
the desired value of `RUSTLIB`, on the oposite, if the `XYZ.bbclass`
is inherit before `cargo.bbclass` then the `RUSTLIB` defined on
`rust-common.bbclass` will prevail.

Changed definition of `RUSTLIB` to soft assignment to make it overridable.

Signed-off-by: Pedro Silva Ferreira <Pedro.Silva.Ferreira@criticaltechworks.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>