armv7: ls1021a: Move SECURE_BOOT option to Kconfig

Move from CONFIG_SYS_EXTRA_OPTIONS to Kconfig option.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/configs/ls1021atwr_nor_SECURE_BOOT_defconfig b/configs/ls1021atwr_nor_SECURE_BOOT_defconfig
index f218e8f..946945f 100644
--- a/configs/ls1021atwr_nor_SECURE_BOOT_defconfig
+++ b/configs/ls1021atwr_nor_SECURE_BOOT_defconfig
@@ -6,7 +6,7 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
-CONFIG_SYS_EXTRA_OPTIONS="SECURE_BOOT"
+CONFIG_SECURE_BOOT=y
 CONFIG_BOOTDELAY=3
 CONFIG_SILENT_CONSOLE=y
 # CONFIG_CONSOLE_MUX is not set