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/p1_p2_rdb_pc.h b/include/configs/p1_p2_rdb_pc.h
index b260129..a9752ad 100644
--- a/include/configs/p1_p2_rdb_pc.h
+++ b/include/configs/p1_p2_rdb_pc.h
@@ -801,7 +801,7 @@
 /*
  * Environment Configuration
  */
-#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 */