ARM: tegra: Unify Tegra186 builds

ARM: tegra: Unify Tegra186 builds

Tegra186 build are currently dealt with in very special ways, which is
because Tegra186 is fundamentally different in many respects. It is no
longer necessary to do many of the low-level programming because early
boot firmware will already have taken care of it.

Unfortunately, separating Tegra186 builds from the rest in this way
makes it difficult to share code with prior generations of Tegra. With
all of the low-level programming code behind Kconfig guards, the build
for Tegra186 can again be unified.

As a side-effect, and partial reason for this change, other Tegra SoC
generations can now make use of the code that deals with taking over a
boot from earlier bootloaders. This used to be nvtboot, but has been
replaced by cboot nowadays. Rename the files and functions related to
this to avoid confusion. The implemented protocols are unchanged.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>