MIPS: kconfig: add options for vct board variant select

The vct board can be built with various variants which are
configured via CONFIG_SYS_EXTRA_OPTIONS. This is deprecated. Thus
add new Kconfig options for those board variants.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Acked-by: Stefan Roese <sr@denx.de>
diff --git a/configs/vct_platinum_onenand_defconfig b/configs/vct_platinum_onenand_defconfig
index 0ba7ae6..4346518 100644
--- a/configs/vct_platinum_onenand_defconfig
+++ b/configs/vct_platinum_onenand_defconfig
@@ -1,4 +1,5 @@
-CONFIG_SYS_EXTRA_OPTIONS="VCT_PLATINUM,VCT_ONENAND"
 CONFIG_MIPS=y
 CONFIG_TARGET_VCT=y
 CONFIG_SYS_BIG_ENDIAN=y
+CONFIG_VCT_PLATINUM=y
+CONFIG_VCT_ONENAND=y