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/pcm051_rev1_defconfig b/configs/pcm051_rev1_defconfig
index d14a51e..f14ba67 100644
--- a/configs/pcm051_rev1_defconfig
+++ b/configs/pcm051_rev1_defconfig
@@ -2,6 +2,7 @@
 CONFIG_TARGET_PCM051=y
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="REV1"
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set