linuxrc: move /etc/initramfs.mounts handling to a separate function
initramfs: use busybox mountpoint rather than copying /bin/mountpoint

/bin/mountpoint was copied to the initramfs only when --multipath
is enabled. Now, I'm not sure why we need the full blown mountpoint
when there is busybox mountpoint which exposes the same cmdline
interface. Furthermore, bb mountpoint can be used to determine if
a filesystem is already mounted.