freescale/powerpc: Rename the config CONFIG_SECURE_BOOT name

Rename the CONFIG_SECURE_BOOT name to CONFIG_NXP_ESBC to avoid
conflicts with UEFI secure boot.

Signed-off-by: Udit Agarwal <udit.agarwal@nxp.com>
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>
diff --git a/configs/T4240QDS_SECURE_BOOT_defconfig b/configs/T4240QDS_SECURE_BOOT_defconfig
index 55a7f19..2c6a7ff 100644
--- a/configs/T4240QDS_SECURE_BOOT_defconfig
+++ b/configs/T4240QDS_SECURE_BOOT_defconfig
@@ -1,6 +1,6 @@
 CONFIG_PPC=y
 CONFIG_SYS_TEXT_BASE=0xEFF40000
-CONFIG_SECURE_BOOT=y
+CONFIG_NXP_ESBC=y
 CONFIG_MPC85xx=y
 CONFIG_TARGET_T4240QDS=y
 # CONFIG_SYS_MALLOC_F is not set