envsetup.sh: cd to toplevel in get_build_var

dumpvar doesn't work right if we're not called from the toplevel;
due to the way the build system works internally, -C does not
suffice. This was already done in get_abs_build_var.

Redundant -C calls removed since we're at the toplevel already.

Change-Id: Iaaa48982547d099186922cc3ddc417a82c85e9a5
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>