scripts/populate: add option to use an alternate sysroot

Add a new command line option, "-r", which allows the user to specify
an alternate sysroot location to copy libraries from. This is useful
when using the toolchain in combination with a separate root filesystem,
or when working with multiple different root filesystems.

Signed-off-by: Nate Case <ncase@xes-inc.com>

reporter_name="Harold Grovesteen"

reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00052.html"

reporter_comment=""