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/hrcon.h b/include/configs/hrcon.h
index d09b863..d6761dd 100644
--- a/include/configs/hrcon.h
+++ b/include/configs/hrcon.h
@@ -578,7 +578,7 @@
 #define CONFIG_LOADADDR	800000	/* default location for tftp and bootm */
 
 
-#define CONFIG_HOSTNAME		hrcon
+#define CONFIG_HOSTNAME		"hrcon"
 #define CONFIG_ROOTPATH		"/opt/nfsroot"
 #define CONFIG_BOOTFILE		"uImage"