v2: Call the build directory 'build/' to clarify that it's a
    directory, not a target (suggestion by Eric Engestrom)

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Daniel Stone <daniels@collabora.com> (v1)
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net> (v1)
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>