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_picon.h b/include/configs/platinum_picon.h
index 2031554..148e561 100644
--- a/include/configs/platinum_picon.h
+++ b/include/configs/platinum_picon.h
@@ -16,7 +16,7 @@
 #define CONFIG_FEC_XCV_TYPE			RMII
 #define CONFIG_FEC_MXC_PHYADDR			0
 
-#define CONFIG_HOSTNAME				picon
+#define CONFIG_HOSTNAME				"picon"
 
 #define CONFIG_PLATFORM_ENV_SETTINGS		"\0"