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/BSC9132QDS.h b/include/configs/BSC9132QDS.h
index ae28868..fb4762d 100644
--- a/include/configs/BSC9132QDS.h
+++ b/include/configs/BSC9132QDS.h
@@ -528,7 +528,7 @@
 #define CONFIG_HAS_ETH1
 #endif
 
-#define CONFIG_HOSTNAME		BSC9132qds
+#define CONFIG_HOSTNAME		"BSC9132qds"
 #define CONFIG_ROOTPATH		"/opt/nfsroot"
 #define CONFIG_BOOTFILE		"uImage"
 #define CONFIG_UBOOTPATH	"u-boot.bin"