README.md: Workaround for setup-environment

Script fsl-community-bsp/setup-environment is currently unable to manage an absolute path for $TOPDIR.

As a workaround, create a link to TOPDIR from the directory which contains setup-environment
($YOCTO/fsl-community-bsp)

Signed-off-by: Gianpaolo Macario <gianpaolo_macario@mentor.com>