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/am335x_baltos_defconfig b/configs/am335x_baltos_defconfig
index 56453a4..9b0f7bc 100644
--- a/configs/am335x_baltos_defconfig
+++ b/configs/am335x_baltos_defconfig
@@ -7,6 +7,7 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_SYS_EXTRA_OPTIONS="NAND"
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set