MIPS: kconfig: add options for dbau1x00 board variant select

The dbau1x00 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>
diff --git a/configs/dbau1500_defconfig b/configs/dbau1500_defconfig
index d2fbff9..d96de13 100644
--- a/configs/dbau1500_defconfig
+++ b/configs/dbau1500_defconfig
@@ -1,4 +1,4 @@
-CONFIG_SYS_EXTRA_OPTIONS="DBAU1500"
 CONFIG_MIPS=y
 CONFIG_TARGET_DBAU1X00=y
 CONFIG_SYS_BIG_ENDIAN=y
+CONFIG_DBAU1500=y