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/C29XPCIE_SPIFLASH_SECBOOT_defconfig b/configs/C29XPCIE_SPIFLASH_SECBOOT_defconfig
index 0c2c0b7..ef1100f 100644
--- a/configs/C29XPCIE_SPIFLASH_SECBOOT_defconfig
+++ b/configs/C29XPCIE_SPIFLASH_SECBOOT_defconfig
@@ -6,7 +6,8 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
-CONFIG_SYS_EXTRA_OPTIONS="SPIFLASH,SECURE_BOOT"
+CONFIG_SYS_EXTRA_OPTIONS="SPIFLASH"
+CONFIG_SECURE_BOOT=y
 CONFIG_BOOTDELAY=-1
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_HUSH_PARSER=y