mpc8548cds: Rework CONFIG_LEGACY usage

This CONFIG option is used in one place, so pick a more direct name and
migrate to Kconfig.  Rework the code slightly.

Cc: Priyanka Jain <priyanka.jain@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/board/freescale/mpc8548cds/Kconfig b/board/freescale/mpc8548cds/Kconfig
index 09f3b0b..87f3374 100644
--- a/board/freescale/mpc8548cds/Kconfig
+++ b/board/freescale/mpc8548cds/Kconfig
@@ -9,4 +9,7 @@
 config SYS_CONFIG_NAME
 	default "MPC8548CDS"
 
+config TARGET_MPC8548CDS_LEGACY
+	bool "Legacy platform support"
+
 endif