linux: disable CONFIG_DEBUG_INFO

This options adds a ton of debug symbols leading to *very* large
binaries:

5.2M kernel-module-8188eu_3.4.79-r0.8_olinuxino_a20.ipk
4.9M kernel-module-8189es_3.4.79-r0.8_olinuxino_a20.ipk
3.6M kernel-module-8192cu_3.4.79-r0.8_olinuxino_a20.ipk

After this change:

244K kernel-module-8188eu_3.4.79-r0.12_olinuxino_a20.ipk
224K kernel-module-8189es_3.4.79-r0.12_olinuxino_a20.ipk
208K kernel-module-8192cu_3.4.79-r0.12_olinuxino_a20.ipk

End users don't need such detailed debug symbols.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>