Fix the EMBEDFS_FORMAT helper variable for riscv64.

It was defined with the wrong MACHINE_ARCH previously. This permits
using an MFS image without defining MD_ROOT_SIZE which has various
benefits (one being that the build is able to treat the MFS image as
a dependency and properly re-link the kernel with the new image when
building with NO_CLEAN).

MFC after: 2 weeks
Sponsored by: DARPA