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/MPC8641HPCN.h b/include/configs/MPC8641HPCN.h
index ff81dd8..9ede6dd 100644
--- a/include/configs/MPC8641HPCN.h
+++ b/include/configs/MPC8641HPCN.h
@@ -602,7 +602,7 @@
 
 #define CONFIG_IPADDR		192.168.1.100
 
-#define CONFIG_HOSTNAME		unknown
+#define CONFIG_HOSTNAME		"unknown"
 #define CONFIG_ROOTPATH		"/opt/nfsroot"
 #define CONFIG_BOOTFILE		"uImage"
 #define CONFIG_UBOOTPATH	u-boot.bin	/* U-Boot image on TFTP server */