The previous commit renamed barebox-pbl-common because you cannot
export a variable that contain hyphens.

These variables are not exported for now. I am just renaming them for
consistency.

  barebox-common -> BAREBOX_OBJS
  barebox-lds    -> BAREBOX_LDS

If barebox moves the link rules into scripts/link-barebox.sh like
Linux did, these two must be exported anyway.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>