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/MPC8313ERDB.h b/include/configs/MPC8313ERDB.h
index 19608b0..ef6d820 100644
--- a/include/configs/MPC8313ERDB.h
+++ b/include/configs/MPC8313ERDB.h
@@ -604,7 +604,7 @@
 
 #define CONFIG_NETDEV		"eth1"
 
-#define CONFIG_HOSTNAME		mpc8313erdb
+#define CONFIG_HOSTNAME		"mpc8313erdb"
 #define CONFIG_ROOTPATH		"/nfs/root/path"
 #define CONFIG_BOOTFILE		"uImage"
 				/* U-Boot image on TFTP server */