Makefile: Invoke dtoc only once

Update the Makefile to run dtoc only once, generating all required files.
This saves time since there is a lot of processing in each invocation of
dtoc.

We already have a variable for the object files to build, so use that
instead of repeating the same filenames. Add a C version of this also,
for the same reason.

This makes it easier to add new C files (generated by dtoc) to the build
later, as needed.

Signed-off-by: Simon Glass <sjg@chromium.org>