data: escape '$' in shell variable assignment

Running bitbake inside make results in the exported environment variable
MAKEOVERRIDES="${-*-command-variables-*-}", which the shell chokes on
when trying to expand it. But of course, it probably shouldn't have been
trying to expand it in the first place -- so just escape the dollar
sign.

Signed-off-by: Richard Tollerton <rich.tollerton@ni.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>