ARM: Distro boot: document the need for fdtfile variable to be set

ARM: Distro boot: document the need for fdtfile variable to be set

When testing builds provided in https://github.com/openwrt/openwrt/pull/3360
I discovered that fdtfile was not set and as a result the firmware was not
functional. So I am documenting what is needed.

Signed-off-by: Dennis Gilmore <dennis@ausil.us>
Cc: Atish Patra <atish.patra@wdc.com>
Cc: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Vagrant Cascadian <vagrant@debian.org>
Cc: Stephen Warren <swarren@nvidia.com>
Cc: Karsten Merker <merker@debian.org>