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/M54418TWR.h b/include/configs/M54418TWR.h
index ba9ba00..c614193 100644
--- a/include/configs/M54418TWR.h
+++ b/include/configs/M54418TWR.h
@@ -85,7 +85,7 @@
 #endif			/* CONFIG_SYS_DISCOVER_PHY */
 #endif
 
-#define CONFIG_HOSTNAME		M54418TWR
+#define CONFIG_HOSTNAME		"M54418TWR"
 
 #if defined(CONFIG_CF_SBF)
 /* ST Micro serial flash */