host-ccache: turn into a proper dependency

This patch moves the host-ccache build target from BASE_TARGETS in Makefile
to an actual host prerequisite in support/dependencies. This causes
host-ccache to be built as part of the dependencies, before any real package
is built.
Since the dependencies are built without ccache anyway, there is no need to
set HOST_CCACHE_CONF_ENV anymore.

Suggested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>