Document manifest change to use groups

Rather than the manifest repo containing separate default.xml and
default-notools.xml, there is now a single default.xml, and one may use
repo init's -g option to cut down what gets synced. Update the README
to document this.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Fix typo in variable name

For some reason (likely cut/paste), all the ArgParser subparsers were
stored in a variable named parser_list_configs. That variable name is at
most appropriate for one subcommand, and in fact the subcommand it would
be relevant for is not implemented by "build" but rather
"tegra-uboot-flasher". Replace this variable name with something more
appropriate and generic.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Add command to import U-Boot binaries from an external location

This option is useful if the user already builds U-Boot in a directory
not managed by the tegra-uboot-flasher manifest. This may happen when
an active U-Boot developer wishes to use the flasher scripts without
relocating their U-Boot development, or when performing separate
automated builds of U-Boot and the flasher, with the flasher build
picking up the result of an abitrary U-Boot build under the control of
the automated build system.

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