buildroot

core: do not reset DEPENDENCIES_HOST_PREREQ in dependencies.mk

Usually, Buildroot does never initialize variables with empty content.
DEPENDENCIES_HOST_PREREQ was an unjustified exception.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>