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/configs/MPC8548CDS_legacy_defconfig b/configs/MPC8548CDS_legacy_defconfig
index 123dd20..8e07df1 100644
--- a/configs/MPC8548CDS_legacy_defconfig
+++ b/configs/MPC8548CDS_legacy_defconfig
@@ -8,9 +8,9 @@
 # CONFIG_CMD_ERRATA is not set
 CONFIG_TARGET_MPC8548CDS=y
 CONFIG_MPC85XX_HAVE_RESET_VECTOR=y
+CONFIG_TARGET_MPC8548CDS_LEGACY=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
-CONFIG_SYS_EXTRA_OPTIONS="LEGACY"
 CONFIG_DYNAMIC_SYS_CLK_FREQ=y
 CONFIG_BOOTDELAY=10
 CONFIG_USE_BOOTCOMMAND=y