targetwide: imagebuilder: add explicit guards around initramfs rules

targetwide: imagebuilder: add explicit guards around initramfs rules

The imagebuilder is not intended to build initramfs images.  Some
profiles attempt to do this and succeed, due to buildroot leaking
the initramfs-kernel into staging_dir; others attempt it, but fail
due to not having initramfs binaries present in the imagebuilder.

Fix this by adding an explict guard around the unsupported generation
of the initramfs images.  This saves space and time during imagebuilder
runs, fixes those that are currently broken and protects against future
breakage for profiles that inadvertently work now.

Fixes: https://github.com/openwrt/openwrt/issues/20151
Signed-off-by: Eric Fahlgren <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20347
Signed-off-by: Robert Marko <[email protected]>