config: kernel: Fix missing symbol on brcm2708 with CGROUPS

config: kernel: Fix missing symbol on brcm2708 with CGROUPS

When CGROUP block io is enabled a new symbol is exposed and needs to
be set or unset else kernel oldconfig hangs waiting for input during
normal OpenWrt builds.  Therefore add sane defaults for this symbol
in that case.  Also, the defaults brcm2708 are different than generic
defaults because the platform's defconfig enables BLK_DEV_THROTTLING
by default (in defconfig config from the patches used to match
upstream's kernel, not in OpenWrt config-4.xx).

Signed-off-by: Daniel F. Dickinson <cshored@thecshore.com>
[make KERNEL_BLK_DEV_THROTTLING_LOW depend on KERNEL_BLK_DEV_THROTTLING]
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>