bloblist: add api to get blob with size

bloblist: add api to get blob with size

bloblist_find function only returns the pointer of blob data,
which is fine for those self-describing data like FDT.
But as a common scenario, an interface is needed to retrieve both
the pointer and the size of the blob data.

Add a few ut test cases for the new api.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>