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/controlcenterd.h b/include/configs/controlcenterd.h
index 5cf9ead..1fe6f2f 100644
--- a/include/configs/controlcenterd.h
+++ b/include/configs/controlcenterd.h
@@ -343,7 +343,7 @@
 
 #else
 
-#define CONFIG_HOSTNAME		controlcenterd
+#define CONFIG_HOSTNAME		"controlcenterd"
 #define CONFIG_ROOTPATH		"/opt/nfsroot"
 #define CONFIG_BOOTFILE		"uImage"
 #define CONFIG_UBOOTPATH	u-boot.bin	/* U-Boot image on TFTP */