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/ve8313.h b/include/configs/ve8313.h
index 3fbac28..bbd8ec5 100644
--- a/include/configs/ve8313.h
+++ b/include/configs/ve8313.h
@@ -437,7 +437,7 @@
 
 #define CONFIG_NETDEV		eth0
 
-#define CONFIG_HOSTNAME		ve8313
+#define CONFIG_HOSTNAME		"ve8313"
 #define CONFIG_UBOOTPATH	ve8313/u-boot.bin
 
 #define CONFIG_EXTRA_ENV_SETTINGS \