kernel: Set CONFIG_RCU_CPU_STALL_TIMEOUT=21

The default value for CONFIG_RCU_CPU_STALL_TIMEOUT was changed from 60
seconds to 21 seconds in 2012 in the upstream kernel. Some targets
already use 21 seconds.

This patch changes the default value in the generic configuration to 21
seconds and removes the target specific configuration options.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Rui Salvaterra <rsalvaterra@gmail.com>