machine: remove MACHINE_ARCH override to fix package collision

Machine-specific packages (like Linux kernel) should have
machine-specific PACKAGE_ARCH that is obtained from MACHINE_ARCH. This
allows to separate their build artifacts in TMPDIR.

Currently all RISC-V machines override MACHINE_ARCH and artifacts for
machine-specific recipes collide. For example kernels of both
qemuriscv64 and freedom-u540 machines will be deployed into the same
$D directory.

Remove MACHINE_ARCH override and set tune PACKAGE_ARCH via
PACKAGE_EXTRA_ARCHS.

Signed-off-by: Taras Kondratiuk <takondra@cisco.com>