bitbake.conf: disable ccache explicitly if it is not enabled
The autogen-native built error on FC17:
ccache: failed to create /dev/null/.ccache
This is because the default gcc command of FC17 is a symlink to ccache,
so the ccache will always be used regardless to the setting of CCACHE,
ccache uses $HOME/.ccache as the CACHE_DIR by default, but autogen set
HOME=/dev/null, so the error happens.
Disable ccache explicitly if it is not enabled would fix the problem,
otherwise it would always use ccache regardless to the setting of CCACHE
on Fedora 17.
The ccache 3.1.7 has a bug, it would always create $CCCHE_DIR/.ccache
even CCACHE_DISABLE=1.
Unset CCACHE_DISABLE in ccache.bbclass, since ccache only checks whether
there is a CCACHE_DISABLE in the environment or not, it doesn't care about
its value, so we need unset it explicitly when enable ccache.
[YOCTO #2554]
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
