cargo_common.bbclass: enable bitbake vendoring for externalsrc

To support crate:// fetcher on externalsrc, we need to remove "-z
${EXTERNALSRC} check of bitbake vendoring. It is possible to disable
vendoring by CARGO_DISABLE_BITBAKE_VENDORING = "1" if externalsrc-ed
project does not want to enablt it.

Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>