kbuild: Add generic rule to apply fdtoverlay

Add a generic rule to apply fdtoverlay in Makefile.lib, so every
platform doesn't need to carry the complex rule. This also automatically
adds "DTC_FLAGS_foo_base += -@" for all base files.

The platform's Makefile only needs to have this now:

 foo-dtbs := foo_base.dtb foo_overlay1.dtbo foo_overlay2.dtbo
 dtb-y := foo.dtb

We don't want to run schema checks on foo.dtb (as foo.dts doesn't exist)
and the Makefile is updated accordingly.

Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Co-developed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Tested-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20920b0df6b067aca4040459a9677d7d1d6d766a.1615354376.git.viresh.kumar@linaro.org