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/MPC8548CDS.h b/include/configs/MPC8548CDS.h
index 7b48135..0c44c16 100644
--- a/include/configs/MPC8548CDS.h
+++ b/include/configs/MPC8548CDS.h
@@ -478,7 +478,7 @@
 
 #define CONFIG_IPADDR	 192.168.1.253
 
-#define CONFIG_HOSTNAME	 unknown
+#define CONFIG_HOSTNAME	 "unknown"
 #define CONFIG_ROOTPATH	 "/nfsroot"
 #define CONFIG_BOOTFILE "8548cds/uImage.uboot"
 #define CONFIG_UBOOTPATH	8548cds/u-boot.bin	/* TFTP server */