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/sbc8349.h b/include/configs/sbc8349.h
index a7c7be7..73a2d19 100644
--- a/include/configs/sbc8349.h
+++ b/include/configs/sbc8349.h
@@ -597,7 +597,7 @@
 #define CONFIG_HAS_ETH1
 #endif
 
-#define CONFIG_HOSTNAME		SBC8349
+#define CONFIG_HOSTNAME		"SBC8349"
 #define CONFIG_ROOTPATH		"/tftpboot/rootfs"
 #define CONFIG_BOOTFILE		"uImage"