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 <[email protected]>
Cc: Atish Patra <[email protected]>
Cc: Lukas Auer <[email protected]>
Cc: Tom Rini <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Vagrant Cascadian <[email protected]>
Cc: Stephen Warren <[email protected]>
Cc: Karsten Merker <[email protected]>