Makefile: remove BASE_TARGETS variable

Because now the toolchain dependency is automatically added by the
package infrastructure the BASE_TARGETS variable is useless so just
remove it.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>