linux-fusion: use a more traditional style to define LINUX_FUSION_MAKE_OPTS

Use a single assignment to define the value of LINUX_FUSION_MAKE_OPTS
instead of a sequence of += assignments.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>