kbuild: move .SECONDARY special target to Kbuild.include

In commit 54a702f7

 ("kbuild: mark $(targets) as .SECONDARY and
remove .PRECIOUS markers"), I missed one important feature of the
.SECONDARY target:

    .SECONDARY with no prerequisites causes all targets to be
    treated as secondary.

... which agrees with the policy of Kbuild.

Let's move it to scripts/Kbuild.include, with no prerequisites.

Note:
If an intermediate file is generated by $(call if_changed,...), you
still need to add it to "targets" so its .*.cmd file is included.

The arm/arm64 crypto files are generated by $(call cmd,shipped),
so they do not need to be added to "targets", but need to be added
to "clean-files" so "make clean" can properly clean them away.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>