fluxbox: uses fork(), not available on noMMU platforms

fluxbox uses the fork() system call, not available on noMMU
platforms. It's also fairly unlikely to have to use a full-blown
X.org window manager on such platforms.

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