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_shc_prompt_defconfig b/configs/am335x_shc_prompt_defconfig
index b9bc355..764fc9f 100644
--- a/configs/am335x_shc_prompt_defconfig
+++ b/configs/am335x_shc_prompt_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SPL=y
 CONFIG_SPL_STACK_R=y
 CONFIG_FIT=y
+CONFIG_VERSION_VARIABLE=y
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_PROMPT="Enter 'shc' to enter prompt (times out) %d \nEnter 'noautoboot' to enter prompt without timeout\n"
 CONFIG_AUTOBOOT_DELAY_STR="shc"