Readme.md: remove positional references from submodule description

Readme.md: remove positional references from submodule description

Remove references to the number of submodules in the tree, as well as
reword the referring to specific submodules as "former" and "previous".
This means we won't need to keep updating the surrounding text if we
add/remove submodules.

Cc: Andrew Fish <afish@apple.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>