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/aristainetos2b.h b/include/configs/aristainetos2b.h
index a680e76..9befb89 100644
--- a/include/configs/aristainetos2b.h
+++ b/include/configs/aristainetos2b.h
@@ -13,7 +13,7 @@
 #define __ARISTAINETOS2B_CONFIG_H
 
 #define CONFIG_SYS_BOARD_VERSION	3
-#define CONFIG_HOSTNAME		aristainetos2
+#define CONFIG_HOSTNAME		"aristainetos2"
 #define CONFIG_BOARDNAME	"aristainetos2-revB"
 
 #define CONFIG_MXC_UART_BASE	UART2_BASE