Update setup script information in the README.md

Update information about all the possible target machines.
Remove extra information about 'evb-ast2500' target as it no longer
needs a special handling.

Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>
Change-Id: Id05bb976cc339d62bcf6433251bd8b0f8787ccbd