km/powerpc: migrate to env.txt file

Use already present common.env file and add a powerpc specific env
so that we can move all the environment defines to text files.

Signed-off-by: Holger Brunck <holger.brunck@hitachienergy.com>
diff --git a/configs/kmsupx5_defconfig b/configs/kmsupx5_defconfig
index 4a02e5e..d981e51 100644
--- a/configs/kmsupx5_defconfig
+++ b/configs/kmsupx5_defconfig
@@ -157,6 +157,7 @@
 CONFIG_MTDPARTS_DEFAULT="mtdparts=boot:768k(u-boot),128k(env),128k(envred),-(ubi0);"
 CONFIG_CMD_UBI=y
 # CONFIG_CMD_UBIFS is not set
+CONFIG_ENV_SOURCE_FILE="km83xx"
 CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_ENV_ADDR_REDUND=0xF00E0000