toolchain/helpers: don't use the x$(...) = x"value" syntax

As noted by Yann E. Morin, the x$(...) = x"value" syntax is old and
ugly, and the easier to read "$(...)" = "value" can now be used
without problems.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>