mpc83xx: Make distinct MPC8313ERDB targets

MPC8313ERDB has the option of either enabling NOR or NAND boot in its
config file (by commenting out certain #ifdefs). To keep this ability
after migrating options to Kconfig, we introduce two MPC8313ERDB
configs: one for NOR, and one for NAND.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/configs/MPC8313ERDB_NAND_33_defconfig b/configs/MPC8313ERDB_NAND_33_defconfig
index 4486571..bcb83b1 100644
--- a/configs/MPC8313ERDB_NAND_33_defconfig
+++ b/configs/MPC8313ERDB_NAND_33_defconfig
@@ -3,7 +3,7 @@
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL=y
 CONFIG_MPC83xx=y
-CONFIG_TARGET_MPC8313ERDB=y
+CONFIG_TARGET_MPC8313ERDB_NAND=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_EXTRA_OPTIONS="SYS_33MHZ"