include/image.mk: call Image/Build/{cpiogz,targz}

All Image/mkfs/... macros call the Image/Build except for
Image/mkfs/targz and Image/mkfs/cpiogz

This patch adds the Image/Build call for Image/mkfs/targz and
Image/mkfs/cpiogz.

Signed-off-by: Maarten Bezemer <m.m.bezemer@utwente.nl>
Signed-off-by: Florian Fainelli <florian@openwrt.org>

SVN-Revision: 34473