configs: Migrate CONFIG_SYS_LDSCRIPT to Kconfig

In order to migrate this symbol to Kconfig introduce a new symbol to
guard it, CONFIG_SYS_CUSTOM_LDSCRIPT.  When that is set we can then
provide the exact final location o the script.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/T1042RDB_PI_NAND_SECURE_BOOT_defconfig b/configs/T1042RDB_PI_NAND_SECURE_BOOT_defconfig
index e0a4d15..ae81f93 100644
--- a/configs/T1042RDB_PI_NAND_SECURE_BOOT_defconfig
+++ b/configs/T1042RDB_PI_NAND_SECURE_BOOT_defconfig
@@ -8,6 +8,8 @@
 CONFIG_SPL=y
 CONFIG_MPC85xx=y
 CONFIG_TARGET_T1042RDB_PI=y
+CONFIG_SYS_CUSTOM_LDSCRIPT=y
+CONFIG_SYS_LDSCRIPT="arch/powerpc/cpu/mpc85xx/u-boot-nand.lds"
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y