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/M5373EVB.h b/include/configs/M5373EVB.h
index 7b2d1bb..faf098e 100644
--- a/include/configs/M5373EVB.h
+++ b/include/configs/M5373EVB.h
@@ -73,7 +73,7 @@
 #	define CONFIG_GATEWAYIP	192.162.1.1
 #endif				/* FEC_ENET */
 
-#define CONFIG_HOSTNAME		M5373EVB
+#define CONFIG_HOSTNAME		"M5373EVB"
 #define CONFIG_EXTRA_ENV_SETTINGS					\
 	"netdev=eth0\0"			\
 	"loadaddr=" __stringify(CONFIG_SYS_LOAD_ADDR) "\0"	\