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/twister.h b/include/configs/twister.h
index 5626eb1..0325596 100644
--- a/include/configs/twister.h
+++ b/include/configs/twister.h
@@ -22,7 +22,7 @@
 
 #define CONFIG_BOOTFILE		"uImage"
 
-#define CONFIG_HOSTNAME twister
+#define CONFIG_HOSTNAME "twister"
 
 #define	CONFIG_EXTRA_ENV_SETTINGS	CONFIG_TAM3517_SETTINGS \
 	"bootcmd=run nandboot\0"