Update README to describe tegrarcm readbct command

Recent versions of tegrarcm now have the capability to read the BCT from the
device without requiring any HW-specific binaries, nor code already installed
on the device. Mention this in the README, since it discusses ways to extract
a BCT from a device.

Also, fix a typo in the Linux MMC block device name that contains the BCT.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Build using Makefiles, add separate bct/img targets

In some situations, one may only need BCT binaries and not flashable
image binaries. Hence, it's useful to allow BCTs and flashable image
to be built separately, rather than always building both together.

While re-writing the build system, convert from shell-scripts to
Makefiles. Now, one can "make", "make bcts", or "make images", all with
appropriate dependency checking.

Many Avionic Design boards use a carrier/module design, with the carrier
and module files stored in separate directories. Use symlinks to handle
this, rather than complicating the build system with support for files
from other directories.

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