kernel-install: fix dracut initrd detection (240 backward compatibility) (#11570)

* kernel-install: fix initrd when called as installkernel

Running make install from the kernel runs e.g.:
installkernel 4.20.5 arch/x86/boot/bzImage System.map "/boot"

Since 0912c0b8 this would
cal 90-loaderentry.install with those arguments:
add 4.20.5 /boot/... arch/x86/boot/bzImage System.map "/boot"

The two last arguments would then be handled as the initrd files.
As System.map exists in current directory but not in /boot/...
it would get copied there, and used as initrd intead of the initrd
which has been generated by dracut.

With this change, nothing changes when kernel-install is called
directly, but when it's called as installkernel, we now pass
thos arguments to 90-loaderentry.install:
add 4.20.5 /boot/... arch/x86/boot/bzImage initrd
initrd is thus detected as the file to use for the initrd, and as it
exists, nothing is copied over and the initrd line generated is
consistent with what one would expect

* kernel-install: fix dracut initrd detection when called directly

This brings back the systemd 240 behaviour when called directly too

* kernel-install: unify initrd fallback

* kernel-install: move initrd fallback handling to 90-loaderentry.install

* kernel-install: move initrd fallback just before creating loader entry