ccache: enable ccache for all recipes if it is in HOSTTOOLS

ccache: enable ccache for all recipes if it is in HOSTTOOLS

At the moment we don't allow ccache for native recipes to avoid the
dependency loop of native recipes needing ccache-native which then
depends on itself.

One way to avoid this dependency loop is to just provide a host ccache,
so enable ccache globally if ccache is in HOSTTOOLS.  This isn't set
out of the box, but does work.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>