ccache: expose control interface via 'make ccache-options'

usage:
	# set cache limit size
	make CCACHE_OPTIONS="--max-size=5G" ccache-options

	# zero statistics counters
	make CCACHE_OPTIONS="--zero-stats" ccache-options

[Peter: drop the redundant ifeq]
Signed-off-by: Tzu-Jung Lee <tjlee@ambarella.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>