cmake: Avoid passing empty prefix to os.path.relpath
With meta-micro, ${prefix} is the empty string. This means that
CMAKE_INSTALL_BINDIR:PATH and friends end up containing paths starting with
many instances of "../", presumably due to os.path.relpath attempting to
find its way to the current directory.
Let's avoid this by ensuring that the root path always ends in a slash. If
it already ends in a slash then adding another one shouldn't cause any
problems.
Signed-off-by: Mike Crowe <mac@mcrowe.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
