ar71xx-generic and -tiny benefit most from the optimized kernel, as they
contain all devices with 32MB RAM. We enable CONFIG_GLUON_SPECIALIZE_KERNEL
for all targets using the mips24_kc architecture so packages shared between
targets don't need to be rebuild all the time.

Fixes #121

Based-on-patch-by: Tobias Hachmer <tobias@hachmer.de>