Makefile: convert "toolchain-eclipse-register" to a hook

For consinstency sake convert the "toolchain-eclipse-register" to a
TARGET_FINALIZE_HOOKS hook.

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