Currently, pbl objects are output to:

  <directory-path>/pbl-<basename>.o

This commit changes as follows:

  <directory-path>/<basename>.pbl.o

The motivation is not only to get rid of the ugly code introduced by
commit 257abdaa36c8 ("Do not rm the path from pbl-y target"), but also
to make it easier to remove obj-dtb-y, pbl-dtb-y, lwl-dtb-y syntax in
the next commit.

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