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/env_default.h b/include/env_default.h
index ba1c3c0..7d5988a 100644
--- a/include/env_default.h
+++ b/include/env_default.h
@@ -75,7 +75,7 @@
 	"netmask="	__stringify(CONFIG_NETMASK)	"\0"
 #endif
 #ifdef	CONFIG_HOSTNAME
-	"hostname="	__stringify(CONFIG_HOSTNAME)	"\0"
+	"hostname="	CONFIG_HOSTNAME	"\0"
 #endif
 #ifdef	CONFIG_BOOTFILE
 	"bootfile="	CONFIG_BOOTFILE			"\0"