Add libtool to dependency list in README

libtools is required by some of the tools that tegra-uboot-flasher can
build. Add it to the list of dependencies.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Use O= rather than BUILD_DIR= when building U-Boot

"make O=" works both before and after U-Boot's conversion to Kbuild,
whereas "make BUILD_DIR=" only works before.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Support U-Boot's temporary output DTB path

Historically, U-Boot has generated $(obj)/u-boot.dtb. During the Kbuild
conversion, this file was moved to $(obj)/dts/dt.dtb. While this change
will be imminentely reverted in the U-Boot makefiles, it's quite easy to
support in the flasher build scripts, so we may as well. This prevents
user confusion and/or "git bisect" issues.

Signed-off-by: Stephen Warren <swarren@nvidia.com>