mk: parallelize make config

mk: parallelize make config

make config dependency resolving was always running serial,
parallelize it for better performance.

$ time make T=x86_64-native-linuxapp-gcc config
real    0m12.633s

$ time make -j8 T=x86_64-native-linuxapp-gcc config
real    0m1.826s

When config creation done under a single make target, using a for loop,
make has no control on the action, and it needs to run as implemented in
the rule. But if for loop converted into multiple targets, make can
detect independent targets and run them parallel based on -j parameter.

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>