dependencies: always use HOSTCC_NOCACHE for DEPENDENCIES_HOST_PREREQ

Currently, HOSTCC and HOSTCXX are set to their _NOCACHE variants in the
'dependencies' target. This is needed because at that time, ccache is
not built yet - host-ccache is one of the dependencies. However, because
this override is only specified for the 'dependencies' target (and
thereby gets inherited by its dependencies), the override is only
applied when the package is reached through the 'dependencies' target.
This is not the case when one of DEPENDENCIES_HOST_PREREQ is built
directly from the command line, e.g. when doing 'make host-ccache'. So
in that case, ccache will be built with ccache... which fails of
course.

To fix this, directly apply the override to the DEPENCIES_HOST_PREREQ
targets.

Note that this only fixes the issue for 'make host-ccache', NOT for
e.g. 'make host-ccache-configure'.

Signed-off-by: Alfredo Alvarez Fernandez <alfredo.alvarez_fernandez@nokia.com>
[Arnout: improve commit message]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>