xtensa: remove meaningless export ccflags-y

arch/xtensa/boot/Makefile does not define ccflags-y at all.

Please do not export ccflags-y because it is meant to be effective
only in the current Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Message-Id: <20200310045925.25396-1-masahiroy@kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>