kbuild: Add top-level target for building gen_init_cpio

kbuild: Add top-level target for building gen_init_cpio

Add a top-level target for building gen_init_cpio to prevent re-entering
kbuild for 'modules-cpio-pkg'.

The recently introduced target 'modules-cpio-pkg' depends on
gen_init_cpio but there is no simple way to add this dependency as a
prerequisite that can be built in parallel to other recipes.

Introducing the top-level target, enables fixing this and also prepares
a possible move of gen_init_cpio from usr/ to scripts/.

Signed-off-by: Nicolas Schier <nsc@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20251128-kbuild-add-top-level-target-for-building-gen_init_cpio-v1-1-84c63a8fc8d4@kernel.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>