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/apf27.h b/include/configs/apf27.h
index e1d4aa6..ceca5ba 100644
--- a/include/configs/apf27.h
+++ b/include/configs/apf27.h
@@ -42,7 +42,7 @@
 #define CONFIG_BOOTP_BOOTFILESIZE
 #define CONFIG_BOOTP_DNS2
 
-#define CONFIG_HOSTNAME	CONFIG_BOARD_NAME
+#define CONFIG_HOSTNAME	"apf27"
 #define CONFIG_ROOTPATH	"/tftpboot/" __stringify(CONFIG_BOARD_NAME) "-root"
 
 /*