User's Guide secton on target hardware setup

Highlight the needs to properly jumper development boards; to
make the OpenOCD configuration match the jumpering; and to have
a usable "reset-init" method when debugging early boot code.

Specific mention of the "ATX Mode" that seems useful on
many i.MX boards, forcing NAND boot.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>