configs: SBx81LIFXCAT: move MTDPART_DEFAULT in defconfig

Replace MTDPARTS_DEFAULT in the config include file by
CONFIG_MTDPARTS_DEFAULT in defconfig to complete the Kconfig migration.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/configs/SBx81LIFXCAT_defconfig b/configs/SBx81LIFXCAT_defconfig
index 1258713..80dbbb1 100644
--- a/configs/SBx81LIFXCAT_defconfig
+++ b/configs/SBx81LIFXCAT_defconfig
@@ -35,6 +35,7 @@
 CONFIG_CMD_PING=y
 CONFIG_CMD_SNTP=y
 # CONFIG_CMD_LED is not set
+CONFIG_MTDPARTS_DEFAULT="mtdparts=spi0.0:768K(boot)ro,256K(boot-env),14M(user),1M(errlog)"
 CONFIG_DOS_PARTITION=y
 CONFIG_OF_CONTROL=y
 CONFIG_ENV_OVERWRITE=y