kernel: enable kernel keyring by default on !SMALL_FLASH

kernel: enable kernel keyring by default on !SMALL_FLASH

Enable CONFIG_KEYS by default on systems which are not marked as
flash-space constraint by the 'small_flash' feature.
CONFIG_KEYS is required by Docker, enabling it in our kernel allows
users to run Docker on stock OpenWrt.
It is also used of by some network file systems (such as NFSv4) to
store credentials as well as UID/GID mappings.

Adds about 50kB to vmlinux on ath79/generic (~18kB compressed)

Signed-off-by: Daniel Golle <daniel@makrotopia.org>