powerpc: mpc85xx: Move SECURE_BOOT to Kconfig

Move from CONFIG_SYS_EXTRA_OPTIONS to Kconfig option.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/configs/T1024QDS_DDR4_SECURE_BOOT_defconfig b/configs/T1024QDS_DDR4_SECURE_BOOT_defconfig
index d7bd23a..bdc0a9d 100644
--- a/configs/T1024QDS_DDR4_SECURE_BOOT_defconfig
+++ b/configs/T1024QDS_DDR4_SECURE_BOOT_defconfig
@@ -7,7 +7,8 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
-CONFIG_SYS_EXTRA_OPTIONS="SYS_FSL_DDR4,SECURE_BOOT"
+CONFIG_SYS_EXTRA_OPTIONS="SYS_FSL_DDR4"
+CONFIG_SECURE_BOOT=y
 CONFIG_BOOTDELAY=10
 CONFIG_SILENT_CONSOLE=y
 # CONFIG_CONSOLE_MUX is not set