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/ma5d4evk.h b/include/configs/ma5d4evk.h
index 4aefc85..a00af1a 100644
--- a/include/configs/ma5d4evk.h
+++ b/include/configs/ma5d4evk.h
@@ -118,7 +118,7 @@
  * Extra Environments
  */
 #define CONFIG_PREBOOT		"run try_bootscript"
-#define CONFIG_HOSTNAME		ma5d4evk
+#define CONFIG_HOSTNAME		"ma5d4evk"
 
 #define CONFIG_EXTRA_ENV_SETTINGS					\
 	"consdev=ttyS3\0"						\