Makefile: generate symbol list from u-boot

Makefile: generate symbol list from u-boot

This information may be useful for both debugging, and processes that want
to perform simple forms of introspection on the U-Boot binary, such as
determining the set of "ut" subtests that are compiled in.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>