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/M54455EVB.h b/include/configs/M54455EVB.h
index 14dd2a5..fb3fee6 100644
--- a/include/configs/M54455EVB.h
+++ b/include/configs/M54455EVB.h
@@ -67,7 +67,7 @@
 #	endif			/* CONFIG_SYS_DISCOVER_PHY */
 #endif
 
-#define CONFIG_HOSTNAME		M54455EVB
+#define CONFIG_HOSTNAME		"M54455EVB"
 #ifdef CONFIG_SYS_STMICRO_BOOT
 /* ST Micro serial flash */
 #define	CONFIG_SYS_LOAD_ADDR2		0x40010013