Kconfig: Escape variables to make in default strings

We have some variables that need to include a variable to pass to make
to evaluate later, typically ARCH and BOARDDIR, to find a file to use.
The way we're doing this today isn't correct but works.  With an update
to Kconfig we will need to escape these properly, so do so.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Tom Rini <trini@konsulko.com>