contrib: Add example cross-build script

contrib: Add example cross-build script

Usage:
export LIBUSB1_SRC=/path/to/libusb-1.0
export OPENOCD_CONFIG="--enable-..."
cd /work/dir
/path/to/openocd/contrib/cross-build.sh <host-triplet>

For static linking, a workaround is to
export LIBUSB1_CONFIG="--enable-static --disable-shared"

All the paths must not contain any spaces.

Feel free to comment or update this patchset with improvements.

Change-Id: Ib3b4970117f13a6140a1eddc493d324a52364519
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1531
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>