ast2500: defconfig: Add Network Block Device

Add NBD support to the defconfig as this will be used to implement
virtual media on the BMC.

This adds about 5.4kB to the kernel image:
Before:
-rw-r--r-- 2 anoo anoo  2542904 Jan 19 14:53 fitImage--4.19.16+git0+bbc22aa295-r0-witherspoon-20190119205057.bin
After:
-rw-r--r-- 2 anoo anoo  2548304 Jan 19 14:59 fitImage--4.19.16+git0+bbc22aa295-r0-witherspoon-20190119205736.bin

Change-Id: I06746eb5dd9ee18ed1dcebc1a250a9554b3ad8db
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/recipes-kernel/linux/ast2500/defconfig b/recipes-kernel/linux/ast2500/defconfig
index 8585efb..9673241 100755
--- a/recipes-kernel/linux/ast2500/defconfig
+++ b/recipes-kernel/linux/ast2500/defconfig
@@ -86,6 +86,7 @@
 CONFIG_MTD_UBI_FASTMAP=y
 CONFIG_MTD_UBI_BLOCK=y
 CONFIG_BLK_DEV_LOOP=y
+CONFIG_BLK_DEV_NBD=y
 CONFIG_ASPEED_LPC_CTRL=y
 CONFIG_ASPEED_LPC_SNOOP=y
 CONFIG_ASPEED_LPC_MBOX=y