busybox: Enable expand(1) utility

Required by perf(1), which is now in the debug tools tarball.

expand(1) expands /bin/busybox.nosuid by 468 bytes and adds one symlink 
to the filesystem:

$ size a/bin/busybox.nosuid 
   text    data     bss     dec     hex filename
 582385    1845    8816  593046   90c96 a/bin/busybox.nosuid
$ size b/bin/busybox.nosuid 
   text    data     bss     dec     hex filename
 582853    1845    8816  593514   90e6a b/bin/busybox.nosuid

Resolves: openbmc/openbmc#2879

Change-Id: I999a12768ab7e0d740bed710b01dab49aa2a8019
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/common/recipes-core/busybox/busybox/busybox.cfg b/common/recipes-core/busybox/busybox/busybox.cfg
index 6afeed5..99e83db 100644
--- a/common/recipes-core/busybox/busybox/busybox.cfg
+++ b/common/recipes-core/busybox/busybox/busybox.cfg
@@ -3,3 +3,4 @@
 CONFIG_FEATURE_LESS_TRUNCATE=y
 CONFIG_FEATURE_LESS_REGEXP=y
 CONFIG_WATCHDOG=y
+CONFIG_EXPAND=y