update-initramfs: Make "-c -k all" options useful again

"update-initramfs -c -k all" used to re-create initramfs images if
they had been deleted without using "update-initramfs -d", as it used
the state directory to determine which kernel versions should have
them.

Since commit f39625af "update-initramfs: Make "-k all" take over other
initramfs images", it ignores the state directory and is almost
equivalent to "update-initramfs -u -k all".

Change it to operate on all installed kernel versions.  This will
ignore which kernel versions suppressed initramfs generation, but
I think it's reasonable to assume users won't use these options
if they have installed such kernels.

Closes: #926401
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>