Makefile: toolchain-eclipse-register: add toolchain dependency

The "toolchain-eclipse-register" target needs the toolchain so add
toolchain as a dependency.
This also fix the support to top-level parallel make.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>