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/MPC8555CDS.h b/include/configs/MPC8555CDS.h
index db4b10e..a74b24c 100644
--- a/include/configs/MPC8555CDS.h
+++ b/include/configs/MPC8555CDS.h
@@ -359,7 +359,7 @@
 
 #define CONFIG_IPADDR    192.168.1.253
 
-#define CONFIG_HOSTNAME  unknown
+#define CONFIG_HOSTNAME  "unknown"
 #define CONFIG_ROOTPATH  "/nfsroot"
 #define CONFIG_BOOTFILE  "your.uImage"