toolchain-wrapper: support change of BR2_CCACHE

By moving the ccache call to the toolchain wrapper, the following
scenario no longer works:

make foo-dirclean all BR2_CCACHE=

That's a sometimes useful call to check if some failure is perhaps
caused by ccache.

We can enable this scenario again by exporting BR_NO_CCACHE when
BR2_CCACHE is not set, and by handling this in the toolchain wrapper.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Romain Naour <romain.naour@openwide.fr>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>