treewide: Convert CONFIG_HOSTNAME to a string option

CONFIG_HOSTNAME is defined as a "plain" preprocessor string, but every
use is couched by __stringify(...).

Hence, convert it to a proper string option.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/include/configs/MPC8569MDS.h b/include/configs/MPC8569MDS.h
index d32a7d7..6d09705 100644
--- a/include/configs/MPC8569MDS.h
+++ b/include/configs/MPC8569MDS.h
@@ -469,7 +469,7 @@
 /*
  * Environment Configuration
  */
-#define CONFIG_HOSTNAME mpc8569mds
+#define CONFIG_HOSTNAME "mpc8569mds"
 #define CONFIG_ROOTPATH  "/nfsroot"
 #define CONFIG_BOOTFILE  "your.uImage"