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/xilinx-ppc440-generic_defconfig b/configs/xilinx-ppc440-generic_defconfig
index 7f085de..a9dd5a7 100644
--- a/configs/xilinx-ppc440-generic_defconfig
+++ b/configs/xilinx-ppc440-generic_defconfig
@@ -6,6 +6,7 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_SYS_EXTRA_OPTIONS="SYS_TEXT_BASE=0x04000000,RESET_VECTOR_ADDRESS=0x04100000,BOOT_FROM_XMD=1"
 CONFIG_BOOTDELAY=5
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="board:/# "
 CONFIG_CMD_ASKENV=y