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/mt_ventoux.h b/include/configs/mt_ventoux.h
index 7109949..3810fd9 100644
--- a/include/configs/mt_ventoux.h
+++ b/include/configs/mt_ventoux.h
@@ -23,7 +23,7 @@
 
 #define CONFIG_BOOTFILE		"uImage"
 
-#define CONFIG_HOSTNAME mt_ventoux
+#define CONFIG_HOSTNAME "mt_ventoux"
 
 /*
  * Set its own mtdparts, different from common