Makefiles: Fix up the powerpc build dirs and include/link paths

The changes for out-of-tree builds are here pretty much the same
as for the ARM Makefile.common.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>