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/platinum_titanium.h b/include/configs/platinum_titanium.h
index 09b9bd3..362d3a4 100644
--- a/include/configs/platinum_titanium.h
+++ b/include/configs/platinum_titanium.h
@@ -21,7 +21,7 @@
 
 #define CONFIG_PHY_RESET_DELAY			1000
 
-#define CONFIG_HOSTNAME				titanium
+#define CONFIG_HOSTNAME				"titanium"
 
 #define CONFIG_PLATFORM_ENV_SETTINGS		"\0"