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/MPC837XERDB.h b/include/configs/MPC837XERDB.h
index 93dc6f0..7ebd617 100644
--- a/include/configs/MPC837XERDB.h
+++ b/include/configs/MPC837XERDB.h
@@ -622,7 +622,7 @@
 
 #define CONFIG_NETDEV		"eth1"
 
-#define CONFIG_HOSTNAME		mpc837x_rdb
+#define CONFIG_HOSTNAME		"mpc837x_rdb"
 #define CONFIG_ROOTPATH		"/nfsroot"
 #define CONFIG_RAMDISKFILE	"rootfs.ext2.gz.uboot"
 #define CONFIG_BOOTFILE		"uImage"