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>

 syntax: glob

 # Generated files

 Makefile

 ct-ng

 docs/ct-ng.1

 docs/ct-ng.1.gz

 paths.mk

 scripts/crosstool-NG.sh

 scripts/saveSample.sh

 scripts/showTuple.sh

 # Temporaries

 .*.swp

 log.*

 # Working files

 kconfig/conf

 kconfig/mconf

 kconfig/**.o

 kconfig/**.dep

 config.gen/

 .config

 .config.old

 targets/