Convert CONFIG_UBIFS_SILENCE_MSG to Kconfig

Introduce another difference from upstream (kernel) source in
fs/ubifs/super.c: adding preprocessor condition as y variable in
mount_ubifs() depends on CONFIG_UBIFS_SILENCE_MSG:
fs/ubifs/super.c:1337:15: error: variable ?y? set but not used [-Werror=unused-but-set-variable]
  long long x, y;

Not setting CONFIG_UBIFS_SILENCE_MSG in am335x_igep003x_defconfig and
igep0032_defconfig. Although it was defined in their config headers, it
depends on CMD_UBIFS which is not set for them.

Signed-off-by: Petr Vorel <petr.vorel@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Heiko Schocher <hs@denx.de>
diff --git a/configs/am335x_igep003x_defconfig b/configs/am335x_igep003x_defconfig
index d2026da..e4292b5 100644
--- a/configs/am335x_igep003x_defconfig
+++ b/configs/am335x_igep003x_defconfig
@@ -39,6 +39,7 @@
 CONFIG_CMD_UBI=y
 # CONFIG_CMD_UBIFS is not set
 CONFIG_ENV_IS_IN_UBI=y
+CONFIG_UBIFS_SILENCE_MSG=y
 CONFIG_MMC_OMAP_HS=y
 CONFIG_NAND=y
 CONFIG_MTD_UBI_FASTMAP=y