build.mk: aarch64: add --nomount to cmdline

don't try to mount zfs in our first simplified loader image.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
XXX add tests/tst-hello.so to the bootfs.manifest.skel

could find no way to do it better.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>