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/novena.h b/include/configs/novena.h
index 977b2c3..121d339 100644
--- a/include/configs/novena.h
+++ b/include/configs/novena.h
@@ -37,7 +37,7 @@
 
 /* Booting Linux */
 #define CONFIG_BOOTFILE			"fitImage"
-#define CONFIG_HOSTNAME			novena
+#define CONFIG_HOSTNAME			"novena"
 
 /* Physical Memory Map */
 #define CONFIG_NR_DRAM_BANKS		1