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>