[SRU trusty] LP#1375310 -- fix initramfs handling with linux-image-extra

[Andy Whitcroft]

We are deleting the initramfs on removal of linux-image-extra instead of
updating it.  What we should do is treat any change to linux-image-extra
the same way we treat _installation_ of linux-image as this always remains
installed.  This issue was introduced when I switched us from a bespoke
postinst/postrm which attempted to do the right thing, but triggered a
rebuild of the wrong initramfs due to trigger handling.

Following this email is a patch which switches us back to a bespoke
postinst/postrm modified to run the appropriate hooks in /etc/kernel
which gains us the correct trigger avoidance for update-initramfstools.

Proposing for SRU to trusty.

-apw