modules: Use posix paths in Kconfig module prompts

kconfiglib doesn't render backslashes in menu strings, and the prompts
in the module menu will be unintelligible on Windows. Print these
menus as posix paths on Windows as well, so they don't look broken in
menuconfig.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>