kernel: add KERNEL_DEVMEM and KERNEL_DEVKMEM

These options are needed to create /dev/mem or /dev/kmem .
/dev/mem is needed by the io tool to access raw hardware memory, which
is helpful when debugging and developing drivers.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: John Crispin <john@phrozen.org>