xtensa: remove unneeded export in boot-elf/Makefile

No one uses these as environment variables.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Message-Id: <20210313122342.69995-2-masahiroy@kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>