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/M54451EVB.h b/include/configs/M54451EVB.h
index b144332..bec0ca0 100644
--- a/include/configs/M54451EVB.h
+++ b/include/configs/M54451EVB.h
@@ -64,7 +64,7 @@
 #	endif			/* CONFIG_SYS_DISCOVER_PHY */
 #endif
 
-#define CONFIG_HOSTNAME		M54451EVB
+#define CONFIG_HOSTNAME		"M54451EVB"
 #ifdef CONFIG_SYS_STMICRO_BOOT
 /* ST Micro serial flash */
 #define	CONFIG_SYS_LOAD_ADDR2		0x40010007