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/MPC8540ADS.h b/include/configs/MPC8540ADS.h
index 0c1c648..a43210f 100644
--- a/include/configs/MPC8540ADS.h
+++ b/include/configs/MPC8540ADS.h
@@ -340,7 +340,7 @@
 
 #define CONFIG_IPADDR    192.168.1.253
 
-#define CONFIG_HOSTNAME		unknown
+#define CONFIG_HOSTNAME		"unknown"
 #define CONFIG_ROOTPATH		"/nfsroot"
 #define CONFIG_BOOTFILE		"your.uImage"