scripts: Makefile.lib: Sanitize DTB names

Having dashes as a separator in the DTB name is a quite common practice.

However, the current code to generate objects from DTBs assumes the
separator is an underscore, leading to a compilation error when building a
device tree with dashes.

Replace all the dashes in the DTB name to generate the symbols name, which
should solve this issue.

Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>