More details on getting Linux sources

Better error message if Linux sources are missing but uClibc tool chain
is being built. Add more info to README on this topic.

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>
Do not do Git operations on directories from source tarball

Decide default value for auto-pull/checkout based on whether toolchain
directory is a Git repository or not. Checkout and pull if it is a
repository and do nothing if it is a directory from a source tarball.

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>