cpio/initramfs: clarify their use in the help text

Recent mails on the list show that it is not very clear how to create
an initial RAM fs with buildroot.  So make this more explicit in the
cpio and initramfs help texts.  Hopefully this will reduce the /init
debugging we have to do.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>