yocto-bsp: Add missing format specifier in bblayers error message

If the build environment is misconfigured (e.g. a bad path
for a layer in bblayers.conf) the yocto-bsp script crashes with a
standard python error, not very explicit.  This fixes the problem.

Signed-off-by: Bastien JAUNY <bastien.jauny@gmail.com>

(From meta-yocto master rev: 4a8e80b812eebdc1c9570b5d88aa0f3b34824b68)

(From meta-yocto rev: 578e06f113d870ec6a4e201458488344ca941e3d)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>