common, kconfig: move VERSION_VARIABLE to Kconfig

move VERSION_VARIABLE from board config file into a
Kconfig option.

Signed-off-by: Heiko Schocher <hs@denx.de>
diff --git a/configs/neo_defconfig b/configs/neo_defconfig
index b61b719..93a402f 100644
--- a/configs/neo_defconfig
+++ b/configs/neo_defconfig
@@ -5,6 +5,7 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_BOOTDELAY=5
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_ELF is not set
 CONFIG_CMD_ASKENV=y