Update dependency documentation to detail the order-only relationship
associated with the DEPENDENCIES variable. See the thread at [1] for
details.
[1] http://lists.busybox.net/pipermail/buildroot/2019-October/262685.html
Signed-off-by: Dan Walkes <danwalkes@trellis-logic.com>
[yann.morin.1998@free.fr: indentation & slight rephrasing]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>