useradd.bbclass: Fix for multilib builds

useradd.bbclass: Fix for multilib builds

The class adds a setscene dependency on base-passwd as well as adds this to DEPENDS.
The DEPENDS version will be auso-converted to include MLPREFIX whilst the setscene
dependency will not. This result in errors about non-existent tasks.

This patch ensures MLPREFIX is added when it is needed and fixes various
build failures. Whether we should have two base-passwd recipes in a multilib
system is a question which would need to be addressed by future changes.

(From OE-Core rev: 22dff7f36e985f9f7275b47e874147bc7bdc9473)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>