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/km/km_arm.h b/include/configs/km/km_arm.h
index 379224d..c676192 100644
--- a/include/configs/km/km_arm.h
+++ b/include/configs/km/km_arm.h
@@ -69,7 +69,7 @@
 		" boardid=0x${IVM_BoardId} hwkey=0x${IVM_HWKey}"
 
 #define CONFIG_KM_DEF_ENV_CPU						\
-	"u-boot="__stringify(CONFIG_HOSTNAME) "/u-boot.kwb\0"		\
+	"u-boot="CONFIG_HOSTNAME "/u-boot.kwb\0"		\
 	CONFIG_KM_UPDATE_UBOOT						\
 	"set_fdthigh=setenv fdt_high ${kernelmem}\0"			\
 	"checkfdt="							\