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/m28evk.h b/include/configs/m28evk.h
index d44cf78..9c4d01e 100644
--- a/include/configs/m28evk.h
+++ b/include/configs/m28evk.h
@@ -97,7 +97,7 @@
 
 /* Extra Environment */
 #define CONFIG_PREBOOT		"run try_bootscript"
-#define CONFIG_HOSTNAME		m28evk
+#define CONFIG_HOSTNAME		"m28evk"
 
 #define CONFIG_EXTRA_ENV_SETTINGS					\
 	"consdev=ttyAMA0\0"						\