buildroot

target/cpio: cleanup

 * Remove the dependency on BR2_HOST_FAKEROOT, since we don't have
   config option for host tools.

 * Remove a few useless things.

 * Check that cpio is available on the host in
   toolchain/dependencies/dependencies.sh.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>