devtool: Fix build-sdk when pn doesn't match filename
If an image with the filename foo.bb could be built using the name "bar"
instead, then build-sdk would fail to create the derivative sdk.
This was because the code assumed that the file name matched the target,
which is not necessarily the case.
Signed-off-by: Randy Witt <randy.e.witt@linux.intel.com> Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
