oxnas: switch CRC32 implementation to SLICEBY8

similar to what was observed on kirkwood this significantly accelerates
btrfs write operations.

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

SVN-Revision: 43498

target/linux/oxnas/config-3.18

@@ -70,6 +70,8 @@ CONFIG_CPU_RMAP=y
 CONFIG_CPU_TLB_V6=y
 CONFIG_CPU_V6K=y
 CONFIG_CRC16=y
+# CONFIG_CRC32_SARWATE is not set
+CONFIG_CRC32_SLICEBY8=y
 CONFIG_CRYPTO_DEFLATE=y
 CONFIG_CRYPTO_LZO=y
 CONFIG_CRYPTO_XZ=y