pitx-imx8m: enable CONFIG_VERSION_VARIABLE config

- This creates environment variable 'ver' that shows the U-Boot
  version the same way it is printed by the 'version' command.
- Note that module release string (e.g. PITX-IMX8M-R10) must be
  introduced to UBOOTRELEASE string by setting and pasting
  CONFIG_LOCALVERSION appropriately during build procedure, e.g.
  echo CONFIG_LOCALVERSION=\"-<release-tag>\" >>.config
Signed-off-by: Thomas Schaefer <thomas.schaefer@kontron.com>

configs/pitx-imx8m_defconfig

@@ -13,6 +13,7 @@ CONFIG_SPL_LOAD_FIT=y
 CONFIG_SPL_FIT_GENERATOR="arch/arm/mach-imx/mkimage_fit_atf.sh"
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/imx8m/imximage.cfg"
+CONFIG_VERSION_VARIABLE=y
 CONFIG_ARCH_MISC_INIT=y
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_USB_HOST_SUPPORT=y