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/MPC8349EMDS.h b/include/configs/MPC8349EMDS.h
index 1452031..c40ba46 100644
--- a/include/configs/MPC8349EMDS.h
+++ b/include/configs/MPC8349EMDS.h
@@ -685,7 +685,7 @@
 #define CONFIG_HAS_ETH0
 #endif
 
-#define CONFIG_HOSTNAME		mpc8349emds
+#define CONFIG_HOSTNAME		"mpc8349emds"
 #define CONFIG_ROOTPATH		"/nfsroot/rootfs"
 #define CONFIG_BOOTFILE		"uImage"